allow tests to optionally use multiple threads

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.
Committed: https://code.google.com/p/skia/source/detail?r=8750

[mtklein, 2013-04-18 18:39:48]

I've got a change out to parallelize test execution at the granularity of a test
(but not inside one as you've got here):
https://codereview.chromium.org/13855007/

I think these extended path ops tests should pick this up for free, and you can
use --threads 0 for the equivalent of --threaded false.  This change might not
be necessary given that one (which also parallelizes the non-path-ops tests)?

[caryclark, 2013-04-18 18:47:43]

Committed patchset #1 manually as r8750 (presubmit successful).