Add tests for restore-mtime

[MestreLion]

restore-mtime is packaged in all major GNU/Linux distributions, for years, sometimes along other tools in this repository. It might have started as a proof-of-concept just to solve a StackOverflow question, but that's no longer the case. If Debian runs in the International Space Station, so can my little baby. It isn't a baby, or even "mine" anymore. it's a big boy now, and as such it deserves to step up.

Tests

Unit tests, coverage tests, CI tests. It needs testing. Any testing is better than current "-v in it's own repo seems fine, and it blazes the linux kernel's repo in about a minute without errors"

I appreciate the help of anyone more experienced in testing than myself. Which basically means anyone and everyone.

Suggestions, patches, PRs. Comments on where to start, or how it should be structured. Everything is welcome.