[UnitTests] More careful removal of fixture functions from module glo…

…bal scope.

- Initial implementation only checked hasattr(obj, "_pytestfixturefunction")
  before removing obj, which gave false positives for objects that implement
  __getattr__, such as caffe.layers.  Now, also check that the value
  contained is a FixtureFunctionMarker.

python/tvm/testing.py

@@ -61,6 +61,7 @@ def test_something():
 import sys
 import time
 import pytest
+import _pytest
 import numpy as np
 import tvm
 import tvm.arith
@@ -1184,7 +1185,9 @@ def _remove_global_fixture_definitions(items):
     for module in modules:
         for name in dir(module):
             obj = getattr(module, name)
-            if hasattr(obj, "_pytestfixturefunction"):
+            if hasattr(obj, "_pytestfixturefunction") and isinstance(
+                obj._pytestfixturefunction, _pytest.fixtures.FixtureFunctionMarker
+            ):
                 delattr(module, name)