[Piglit] Fix subtest handling in summary groups other than 'all'

[baker.dylan.c at gmail.com]

Currently we handle the 'all' group correctly, tests, groups, and
subtests are handled as expected, groups are groups, tests with subtests
are groups, tests without subtests are tests, and subtests are tests.
This behavior is not carried over to the other groups (regressions,
disabled, changes, etc).

This series corrects that, by building helpers that make handling
subtests easier, adding tests to demonstrate the problem, and finally
fixing the problem.

Marek: This should fix the problem you're seeing in the problems page of
the html summary. If you could test this and make sure it would be much
appreciated.