[Piglit] Fix subtests

[Dylan Baker]

This series attempts to fix the way piglit handles subtests. In
particular there are two problems with subtests:
 1. A subtest failure does not cause the test it is a part of to be a
    failure
 2. Each subtest writes an additional copy of itself in the json

Patch 1 attempts to correct point 1
The primary problem with tests not failing when their subtests fail is
that it is inconsistent with the rest of piglit; a group's status is
that of it's lowest test or subgroup.

Patch 2 corrects point 2
Since tests with subtests are now groups, there is no need to write an
entry for each subtests. This reduces the size the json generated by
~80% (That's 1/5 of the original size). This is significant not just for
the space savings, but because writing files is expensive.