This PR ensures that all tests are run in WebAssembly except for the credentials tests, support for which will follow in another PR.

Summary of changes:

1. Add the 4 test files not already covered (diff, fetch, push and tag).
2. Add monkeypatched functions os.rename, os.symlink and Path.write_bytes.
3. Rearrange the monkeypatched functions to satisfy linter.
4. New test function strip_ansi_colours which removes ANSI colour sequences from a string. This is needed because by default the normal linux/macos tests do not include colour sequences but the wasm ones do, so tests that check the contents of stdout returned by git2cpp functions need to remove colour sequences that are added. In future I am intending that we have some tests with and some without colour sequences for both the normal and wasm tests.
5. Remove passing env in subprocess.run as the commit_env_config fixture does this automatically.