introduce an option that avoids discovery of classes other than unittest.TestCase in modules

importing unittest.

testing/plugin/test_assertion.py

@@ -161,6 +161,15 @@ def test_functional(testdir):
     result = testdir.runpytest("--no-assert")
     assert "3 == 4" not in result.stdout.str()
 
+def test_AssertionErrorIdentity(testdir):
+    testdir.makepyfile("""
+        def test_hello():
+            import exceptions
+            assert AssertionError is exceptions.AssertionError
+    """)
+    result = testdir.runpytest()
+    result.stdout.fnmatch_lines(["*1 passed*"])
+
 def test_triple_quoted_string_issue113(testdir):
     testdir.makepyfile("""
         def test_hello():