add parse_match tests for atoms with subslots
simplify similar exception test cases for parse_match
add support for "*/*::repo" wildcard patterns for atoms

Pkgcore already supported "*::repo" but that isn't supported by portage
while the "*/*::repo" format is. This allows cross-compliance for people
using similar patterns in their config files to mask, unmask, or keyword
entire repos or for other related usage.
simplify duplicate conditional case for parse_match