GPUs like to hang, especially when barraged with lots of mean Piglit
tests.  Usually this results in the poor developer having to figure out
what test hung, blacklist it via -x, and start the whole test run over.
This can waste a huge amount of time, especially when many tests hang.

This patch adds the ability to resume a Piglit run where you left off.

The workflow is:
$ piglit-run.py -t foo tests/quick.tests results/foobar-1
<interrupt the test run somehow>
$ piglit-run.py -r -x bad-test results/foobar-1

To accomplish this, piglit-run.py now stores the test profile
(quick.tests) and -t/-x options in the JSON results file so it can tell
what you were originally running.  When run with the --resume option, it
re-reads the results file to obtain this information (repairing broken
JSON if necessary), rewrites the existing results, and runs any
remaining tests.

WARNING:
Results files produced after this commit are incompatible with older
piglit-summary-html.py (due to the extra "option" section.)

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>