The multicontext test could spam thousands of lines of error text
when the test was broken. The test will still produce a lot of
errors when everything is broken, but it will produce a lot less.

BUG=angleproject:2088

Change-Id: Ia07e28897de3aba28cc8468acf27d5c9f56193a4
Reviewed-on: https://chromium-review.googlesource.com/549979Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>