Add support of TEST_TAGS in python tests (elastic#17075)

Add a `tag(tag)` decorator that skips a test if the tag is not included
in the comma-separated list of `TEST_TAGS` environment variable.
This offers an initial support for the similar implementation added for
mage in elastic#16937.

(cherry picked from commit 3bf05f5)

libbeat/tests/system/beat/tags.py

@@ -0,0 +1,25 @@
+import os
+import unittest
+
+
+def tag(tag):
+    """
+    Decorates a test function with a tag following go build tags semantics,
+    if the tag is not included in TEST_TAGS environment variable, the test is
+    skipped.
+    TEST_TAGS can be a comma-separated list of tags, e.g: TEST_TAGS=oracle,mssql.
+    """
+    def decorator(func):
+        def wrapper(*args, **kwargs):
+            set_tags = [
+                tag.strip() for tag in os.environ.get("TEST_TAGS", "").split(",")
+                if tag.strip() != ""
+            ]
+            if not tag in set_tags:
+                raise unittest.SkipTest("tag '{}' is not included in TEST_TAGS".format(tag))
+            return func(*args, **kwargs)
+        wrapper.__name__ = func.__name__
+        wrapper.__doc__ = func.__doc__
+        return wrapper
+
+    return decorator

metricbeat/tests/system/metricbeat.py

@@ -1,11 +1,12 @@
+import os
 import re
 import sys
-import os
 import yaml
 
 sys.path.append(os.path.abspath(os.path.join(os.path.dirname(__file__), '../../../libbeat/tests/system')))
 
 from beat.beat import TestCase
+from beat.tags import tag
 from parameterized import parameterized_class
 
 COMMON_FIELDS = ["@timestamp", "agent", "metricset.name", "metricset.host",