Add automated high-level tests (pre-commit/CLI)

[apirogov]

As pre-commit is a recommended way of using somesy, we should also add some basic tests to make sure the hook works correctly.

The test could be something like:

• Initialize a git repo in a temp dir with some test contents and with somesy as pre-commit hook
• change metadata (change somesy.toml or one of the target files)
• use pre-commit to check the repo and make sure somesy works as expected (updates files -> "fail")
• check that after adding the updated changes to the commit, the commit will succeed (somesy will "pass")