gh-93839: Move Lib/unttest/test/ to Lib/test/test_unittest/ (#94043)

* Move Lib/unittest/test/ to Lib/test/test_unittest/
* Remove Lib/test/test_unittest.py
* Replace unittest.test with test.test_unittest
* Remove unittest.load_tests()
* Rewrite unittest __init__.py and __main__.py
* Update build system, CODEOWNERS, and wasm_assets.py

Lib/test/test_unittest/testmock/support.py

@@ -0,0 +1,16 @@
+target = {'foo': 'FOO'}
+
+
+def is_instance(obj, klass):
+    """Version of is_instance that doesn't access __class__"""
+    return issubclass(type(obj), klass)
+
+
+class SomeClass(object):
+    class_attribute = None
+
+    def wibble(self): pass
+
+
+class X(object):
+    pass