Fix regular expressions in return_value_policy_test.
Fix regular expressions in return_value_policy_test.

The test case is using `assertRegex(str, regex)`, which asserts the regular expression can be found in the string (or substring). This is different from what we want. We need to assert the string matches the regex completely.

Also make the test run with generated pybind11 module.

PiperOrigin-RevId: 367710613

google/clif,google/clif,google/clif