Introduce ad hoc requirements and add skip_without method to optdeps

Introduce ad hoc requirements and add skip_without method to optdeps

On rare occasions tests need a specific module version, but nothing else in
the dist needs this. Additionally only parts of a test may rely on this extra
requirement, making the "all or nothing" approach of -skip_all_without not
very attractive.

Hence introducing ability to supply arbitrary module specifications in lieu of
group names. Each specification is checked against a special group 'test_adhoc'
which ensures that optional deps are not "forgotten" within the suite (causing
a test to never run in practice).