Allow custom suffix/identifier for @TEST-START-NEXT

[awelzel]

This is motivated by: https://github.com/zeek/zeekctl/pull/84/files#r2218909229

Using @TEST-START-NEXT is a valid suggestion, but then it produces baselines and testing output with a non-descriptive -2 suffix. It'd be nice if @TEST-START-NEXT could be given an optional identifier such that instead of -2 it'd end with -<identifier>. Supposedly all @TEST-START-NEXT lines in one test file should've identifiers and otherwise error to prevent mixing the different styles. Duplicated identifiers would also be an error.

This would help test documentation and also aid working with baselines a bit: Trying to map baselines produced by @TEST-START-NEXT to the right snippets or vice-versa is pretty painful now that I remember back to the few occasions I had to do so.