Expose 'node name' into context

[lukaszachy]

Allow

expression ::= 'true' | 'false' | '/node/name/regexp'

Expected usage is for adjust-tests (https://tmt.readthedocs.io/en/stable/plugins/discover.html#configuration) to be able to apply rules only to selected nodes.

Todo: Find syntax for 'negative' -> when node name doesn't match.

[LecrisUT]

You mean as in allowing

adjust:
  when: /tests/foo_.*
  ...

I guess this could be more concise than #227 (PS: path in that issue should be name I was thinking too much in the context of #206 at that time), but it would be nice to also extract regex groups from that matcher.

How do we separate the literal true/false from the regex pattern? We take the former to have priority?