The MatchOutputCodeTest class makes it easier to implement tests that
do string matching on compiler output. Inheriting test classes set the
compiler settings, tests then call compile() with the shader string
and can call foundInCode() to check if the output code contains a
given string.

Various compiler unit tests that already did string matching are
refactored to make use of this new helper class. Some tests now use
SH_GLES3_SPEC instead of SH_GLES2_SPEC - this should not have a
significant impact on test coverage.

Some compileTestShader function variants that are now unused can be
removed from the code.

BUG=angleproject:1430
TEST=angle_unittests

Change-Id: I1fd3529d5a1c6ab192f95ace800cf162604e68e7
Reviewed-on: https://chromium-review.googlesource.com/357800Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>