Allow optional targets to specify tests.

Add sources_for_tests to optional targets. If set, a taget_name_tests
source_set will be created which matches the target_name target except
that the sources are the sources_for_tests.

This allows the tests for these targets to be obviously related to the
target they test instead of having to diplicate the logic of the
optional's enabled predicate in the test target. The test target can
just directly depend on the optional test target in the same way it can
depend directly on module tests.

This was motivated by the existence of two such tests and the need to
soon introduce another (for CoreText).

Change-Id: Idc6138044ade063f47a763f14aa7a406e613af26
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/371481
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
2 files changed