Add test to ensure _pytest is warning-clean on import

Author: asottile

testing/test_meta.py

@@ -0,0 +1,28 @@
+import pkgutil
+import subprocess
+import sys
+
+import _pytest
+import pytest
+
+
+def _modules():
+    return sorted(
+        n
+        for _, n, _ in pkgutil.walk_packages(
+            _pytest.__path__, prefix=_pytest.__name__ + "."
+        )
+    )
+
+
+@pytest.mark.parametrize("module", _modules())
+def test_no_warnings(module):
+    # fmt: off
+    subprocess.check_call((
+        sys.executable,
+        "-W", "error",
+        # https://github.com/pytest-dev/pytest/issues/5901
+        "-W", "ignore:The usage of `cmp` is deprecated and will be removed on or after 2021-06-01.  Please use `eq` and `order` instead.:DeprecationWarning",  # noqa: E501
+        "-c", "import {}".format(module),
+    ))
+    # fmt: on