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

Author: vstinner

.github/CODEOWNERS

@@ -97,7 +97,7 @@ Lib/ast.py                    @isidentical
 
 # Mock
 /Lib/unittest/mock.py         @cjw296
-/Lib/unittest/test/testmock/* @cjw296
+/Lib/test/test_unittest/testmock/* @cjw296
 
 # SQLite 3
 **/*sqlite*                   @berkerpeksag @erlend-aasland

Lib/test/test_unittest.py

@@ -0,0 +1,6 @@
+import os.path
+from test.support import load_package_tests
+
+
+def load_tests(*args):
+    return load_package_tests(os.path.dirname(__file__), *args)

Lib/test/test_unittest/__init__.py

@@ -0,0 +1,4 @@
+from . import load_tests
+import unittest
+
+unittest.main()