refactor: split pandas system tests to new module (#548)

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [ ] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/python-bigquery/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)

Follow-up to #448

Towards #366

tests/system/conftest.py

@@ -0,0 +1,39 @@
+# Copyright 2021 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import pytest
+
+from . import helpers
+
+
+@pytest.fixture(scope="session")
+def bigquery_client():
+    from google.cloud import bigquery
+
+    return bigquery.Client()
+
+
+@pytest.fixture(scope="session")
+def bqstorage_client(bigquery_client):
+    from google.cloud import bigquery_storage
+
+    return bigquery_storage.BigQueryReadClient(credentials=bigquery_client._credentials)
+
+
+@pytest.fixture
+def dataset_id(bigquery_client):
+    dataset_id = f"bqsystem_{helpers.temp_suffix()}"
+    bigquery_client.create_dataset(dataset_id)
+    yield dataset_id
+    bigquery_client.delete_dataset(dataset_id, delete_contents=True)

tests/system/helpers.py

@@ -0,0 +1,94 @@
+# Copyright 2021 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import datetime
+import decimal
+import uuid
+
+import google.api_core.exceptions
+import test_utils.retry
+
+from google.cloud._helpers import UTC
+
+
+_naive = datetime.datetime(2016, 12, 5, 12, 41, 9)
+_naive_microseconds = datetime.datetime(2016, 12, 5, 12, 41, 9, 250000)
+_stamp = "%s %s" % (_naive.date().isoformat(), _naive.time().isoformat())
+_stamp_microseconds = _stamp + ".250000"
+_zoned = _naive.replace(tzinfo=UTC)
+_zoned_microseconds = _naive_microseconds.replace(tzinfo=UTC)
+_numeric = decimal.Decimal("123456789.123456789")
+
+
+# Examples of most data types to test with query() and DB-API.
+STANDARD_SQL_EXAMPLES = [
+    ("SELECT 1", 1),
+    ("SELECT 1.3", 1.3),
+    ("SELECT TRUE", True),
+    ('SELECT "ABC"', "ABC"),
+    ('SELECT CAST("foo" AS BYTES)', b"foo"),
+    ('SELECT TIMESTAMP "%s"' % (_stamp,), _zoned),
+    ('SELECT TIMESTAMP "%s"' % (_stamp_microseconds,), _zoned_microseconds,),
+    ('SELECT DATETIME(TIMESTAMP "%s")' % (_stamp,), _naive),
+    ('SELECT DATETIME(TIMESTAMP "%s")' % (_stamp_microseconds,), _naive_microseconds,),
+    ('SELECT DATE(TIMESTAMP "%s")' % (_stamp,), _naive.date()),
+    ('SELECT TIME(TIMESTAMP "%s")' % (_stamp,), _naive.time()),
+    ('SELECT NUMERIC "%s"' % (_numeric,), _numeric),
+    ("SELECT (1, 2)", {"_field_1": 1, "_field_2": 2}),
+    (
+        "SELECT ((1, 2), (3, 4), 5)",
+        {
+            "_field_1": {"_field_1": 1, "_field_2": 2},
+            "_field_2": {"_field_1": 3, "_field_2": 4},
+            "_field_3": 5,
+        },
+    ),
+    ("SELECT [1, 2, 3]", [1, 2, 3]),
+    (
+        "SELECT ([1, 2], 3, [4, 5])",
+        {"_field_1": [1, 2], "_field_2": 3, "_field_3": [4, 5]},
+    ),
+    (
+        "SELECT [(1, 2, 3), (4, 5, 6)]",
+        [
+            {"_field_1": 1, "_field_2": 2, "_field_3": 3},
+            {"_field_1": 4, "_field_2": 5, "_field_3": 6},
+        ],
+    ),
+    (
+        "SELECT [([1, 2, 3], 4), ([5, 6], 7)]",
+        [{"_field_1": [1, 2, 3], "_field_2": 4}, {"_field_1": [5, 6], "_field_2": 7}],
+    ),
+    ("SELECT ARRAY(SELECT STRUCT([1, 2]))", [{"_field_1": [1, 2]}]),
+    ("SELECT ST_GeogPoint(1, 2)", "POINT(1 2)"),
+]
+
+
+def temp_suffix():
+    now = datetime.datetime.now()
+    return f"{now.strftime('%Y%m%d%H%M%S')}_{uuid.uuid4().hex[:8]}"
+
+
+def _rate_limit_exceeded(forbidden):
+    """Predicate: pass only exceptions with 'rateLimitExceeded' as reason."""
+    return any(error["reason"] == "rateLimitExceeded" for error in forbidden._errors)
+
+
+# We need to wait to stay within the rate limits.
+# The alternative outcome is a 403 Forbidden response from upstream, which
+# they return instead of the more appropriate 429.
+# See https://cloud.google.com/bigquery/quota-policy
+retry_403 = test_utils.retry.RetryErrors(
+    google.api_core.exceptions.Forbidden, error_predicate=_rate_limit_exceeded,
+)