enable test_string and update test notes

from_cpython/Lib/test/test_string.py

-# skip-if: True
-# expected: fail
-# this test gets stuck in an infinite loop(?) in test_replace (from string_tests)
 import unittest, string
 from test import test_support, string_tests
 from UserList import UserList

test/CPYTHON_TEST_NOTES.md

@@ -200,7 +200,6 @@ test_sort               argument specification issue in listSort?
 test_sqlite             [unknown]
 test_ssl                [unknown]
 test_startfile          [unknown]
-test_string             infinite loops in test_replace
 test_str                memory leak?
 test_strtod             [unknown]
 test_structmembers      [unknown]