Tests: Added new pattern check for octal escapes

[RunDevelopment]

This adds a new pattern check to disallow octal character escapes (e.g. \12). Octal escapes are not allowed in Unicode mode and for good reasons. They look like back-references and are a potential cause for problems because of it. E.g. \2 can back-reference or the octal escape for \x02 depending on how many capturing groups the pattern has. This is a problem when (re-)writing patterns as removing a capturing group can break the back-reference but will still result in a valid regex.

I even found a pattern with this problem. My own.
In #1892, I copied the here-doc string pattern from bash and probably removed a capturing group after verifying that everything works breaking my pattern.
This problem is no more and I also remove an unnecessary non-capturing group in bash.