build: Split perfetto_unittests into sources + executable
build: Split perfetto_unittests into sources + executable

Chrome can then specify their own test target.

Change-Id: I76580e666c631f8fc0b91a0722ac6e0cf1f14007
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto