allow tests to optionally use multiple threads

modify threaded path ops tests to check

Background: this CL came out of a conversation with Eric where I learned that 10s of machines host 100s of bots. Since the bot hosting tests may be shared with many other tasks, it seems unwise for path ops to launch multiple test threads.

The change here is to make launching multiple threads "opt-in" and by default, bots can run path ops in a single thread.
Review URL: https://codereview.chromium.org/14002007

git-svn-id: http://skia.googlecode.com/svn/trunk@8750 2bbb7eff-a529-9590-31e7-b0007b416f81
12 files changed