Is your feature request related to a problem? Please describe.
Playing around with tags, for me it is often much easier to tag several problematic (or long-running) tests which I don't want to run in several situtations.

Describe the solution you'd like
Would be great to enable ut-runner Tags-parameter to allow -tag:

begin
  ut.run(a_tags=>'-long_running');
end;

This would run all tests which don't have the long_running tag.

begin
  ut.run(a_tags=>'wip,-long_running');
end;

This would run all tests that have the wip tag but not the long_running tag

Additional context
We discussed an alternative approach with logic AND/OR operators (a_tags => 'tag1&tag2|tag3) but the - seems much more useful.