Refactored test targets into angle_unittests and angle_end2end_tests.

angle_unittests subsumes the previous:
  angle_compiler_tests
  angle_preprocessor_tests
and will subsume angle_implementation_unit_tests in a follow-on CL.

angle_end2end_tests subsumes the previous:
  angle_tests
  angle_standalone_tests
  angle_implementation_unit_tests

The bulk of these two targets have been factored into their own
.gypi files, so that a different main.cpp can be trivially
plugged in for the gtest harness. This refactoring has been
tested both in ANGLE standalone builds, and builds within
Chromium.

BUG=chromium:435726

Change-Id: I231a3c1989c17e188cc469dcf80fe78b052afe78
Reviewed-on: https://chromium-review.googlesource.com/236681
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Kenneth Russell <kbr@chromium.org>