Add tests for sub-interpreter support

These are flaky as they use internal CPython packages, but that will do
until the "interpreters" module is made eventually official.

Signed-off-by: Rodrigo Tobar <rtobar@icrar.org>

test/test_subinterpreter.py

@@ -0,0 +1,47 @@
+import contextlib
+from typing import Iterable, Tuple, no_type_check
+
+import pytest
+
+import crc32c
+
+try:
+    from test.support.interpreters import Interpreter  # type: ignore
+    from test.support.interpreters import create, queues
+except ImportError:
+    pytest.skip("No sub-interpreter support", allow_module_level=True)
+
+
+@pytest.fixture(name="interpreter")
+def interpreter_fixture() -> Iterable[Interpreter]:
+    interpreter = create()
+    with contextlib.closing(interpreter):
+        yield interpreter
+
+
+def test_crc32c_can_be_loaded(interpreter: Interpreter) -> None:
+    interpreter.exec("import crc32c")
+    # all good, no exception raised
+
+
+@pytest.mark.calculates_crc32c
+def test_crc32c_can_run(interpreter: Interpreter) -> None:
+
+    queue = queues.create()
+    VALUE = b"test"
+    interpreter.prepare_main(queue_id=queue.id, value_in=VALUE)
+
+    @no_type_check
+    def crc32c_in_subinterpreter() -> None:
+        from test.support.interpreters.queues import Queue  # type: ignore
+
+        import crc32c
+
+        queue = Queue(queue_id)
+        value_out = crc32c.crc32c(value_in)
+        queue.put(value_out)
+
+    thread = interpreter.call_in_thread(crc32c_in_subinterpreter)
+    result = queue.get(timeout=5)
+    thread.join()
+    assert result == crc32c.crc32c(VALUE)