Reorganize hwui test directories

Now test-only common files reside in tests/common, and each test
executable type (macrobench, microbench, and unit) has a subdir there.

This change means the shared lib no longer has test code in it, and
sets up a means for scenes to be shared between tests.

Change-Id: I37b081f6977300e03fdd961b8e6439fde730605e