Rename and slightly reorganize test targets.

Use "angle_" prefix for all targets to disambiguate them from any
potential Chromium-side targets.

Combine compiler and preprocessor tests into angle_compiler_tests.
(Chromium's "angle_unittests" target contains both.) Deleted
preprocessor_test_main.cpp.

BUG=angleproject:833

Change-Id: I7acc5a76f49c15d0064c16d2ace256f9cc86e9e8
Reviewed-on: https://chromium-review.googlesource.com/231185
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Kenneth Russell <kbr@chromium.org>
Fixed Mac and Linux build breakage.

Introduced in 1edac3b1bdcc54a4b0f0449cb727bda84ab3ee53 .

BUG=none

Change-Id: Ia5c5e38b794e9c991ec8624de34681dcf13a651f
Reviewed-on: https://chromium-review.googlesource.com/231441
Tested-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>