Add file test with regexp

[jpmens]

I mentioned this recently on the Fediverse, and now as a reminder. What I'd like to see is for simplomon being able to verify whether a particular file is on the file system, and check preferable via a regexp I specify if a string is contained therein.

Regexp check can also be useful on, say, https checks to determine whether a particular string or expression is contained in whatever the check responds with.