Split the :tests target into a :test target which contains the test
framework and :tests which collects all of the test cases. This allows
for all targets which define tests to depend on :test in order to define
tests, with :tests then depending on all targets which define tests. A
similar split should be considered for gms, samples, and benches.
Change-Id: Ic9f373ec0c1a8ea842fa68327e854db23477caae
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/371696
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
bded42ab51