Revised the test suite for 'contains' to use the test() function argument

rather than vereq().  While it was effectively testing regular strings, it
ignored the test() function argument when called by test_userstring.py.

Lib/test/string_tests.py

@@ -305,12 +305,12 @@ def test_exception(lhs, rhs, msg):
         raise TestFailed, msg
 
 def run_contains_tests(test):
-    vereq('' in '', True)
-    vereq('' in 'abc', True)
-    vereq('\0' in 'abc', False)
-    vereq('\0' in '\0abc', True)
-    vereq('\0' in 'abc\0', True)
-    vereq('a' in '\0abc', True)
-    vereq('asdf' in 'asdf', True)
-    vereq('asdf' in 'asd', False)
-    vereq('asdf' in '', False)
+    test('__contains__', '', True, '')         # vereq('' in '', True)
+    test('__contains__', 'abc', True, '')      # vereq('' in 'abc', True)
+    test('__contains__', 'abc', False, '\0')   # vereq('\0' in 'abc', False)
+    test('__contains__', '\0abc', True, '\0')  # vereq('\0' in '\0abc', True)
+    test('__contains__', 'abc\0', True, '\0')  # vereq('\0' in 'abc\0', True)
+    test('__contains__', '\0abc', True, 'a')   # vereq('a' in '\0abc', True)
+    test('__contains__', 'asdf', True, 'asdf') # vereq('asdf' in 'asdf', True)
+    test('__contains__', 'asd', False, 'asdf') # vereq('asdf' in 'asd', False)
+    test('__contains__', '', False, 'asdf')    # vereq('asdf' in '', False)