[KopiLua] Using targetFramework 4.5 to run tests.
Backport of 5.2.2 string library

Fixed StackOverflowException
[KopiLua] cherry-picked from Changed string.format to use the InvariantCulture (from @JCH2k) https://github.com/mlnlover11/KopiLua-v5.1.5/commit/9f068a4f505ea77d04dd443125817dc62a435350