Add --threads to tests binary, to run non-GPU tests on multiple cores.

Add --threads to tests binary, to run non-GPU tests on multiple cores.

On my quad-core laptop I can get about a 3x speedup:
  Debug,   --threads 0  40.99s
  Debug,   --threads 8  14.39s
  Release, --threads 0   8.24s
  Release, --threads 8   2.80s

I also removed some unused Test.{h,cpp} APIs and refactored a little to make
things thread-safer.

BUG=
R=borenet@google.com, djsollen@google.com, scroggo@google.com, reed@google.com

Author: mtklein@google.com

Review URL: https://chromiumcodereview.appspot.com/13855007

git-svn-id: http://skia.googlecode.com/svn/trunk@8763 2bbb7eff-a529-9590-31e7-b0007b416f81