pythongh-89392: Make test_pep646_syntax discoverable (pythonGH-108861)

(cherry picked from commit d0b22f6)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Author: serhiy-storchaka

Lib/test/test_pep646_syntax.py

@@ -1,3 +1,5 @@
+import doctest
+
 doctests = """
 
 Setup
@@ -317,10 +319,10 @@
 
 __test__ = {'doctests' : doctests}
 
-def test_main(verbose=False):
-    from test import support
-    from test import test_pep646_syntax
-    return support.run_doctest(test_pep646_syntax, verbose)
+def load_tests(loader, tests, pattern):
+    tests.addTest(doctest.DocTestSuite())
+    return tests
+
 
 if __name__ == "__main__":
-    test_main(verbose=True)
+    unittest.main()