ruby-ng.eclass: cater to USE_RUBY conditional calls

Packages may try to restrict their test dependencies to ease bootstrapping/porting
if they're not yet available for a newer Ruby implementation by setting
USE_RUBY="<some subset of original USE_RUBY>" ruby_add_bdepend ...

For example, dev-ruby/parallel does:
```
 # Rails isn't yet ruby32-ready in Gentoo
 USE_RUBY="ruby27 ruby30 ruby31" ruby_add_bdepend "
         test? ( dev-ruby/activerecord[sqlite] )
```

This isn't ideal, but we don't have a ruby_gen_cond_dep right now, so
cater to this usecase by invalidating the caches we've previously added
if we've detected USE_RUBY changed.

Signed-off-by: Sam James <sam@gentoo.org>

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