autotest: Allow crash collection to be skipped

Expose this functionality as a flag that defaults to collection crashes
as normal.  This makes test runs faster for developers iterating a
test/patch.

BUG=chromium:250366
TEST=Ran a test, observed that collection was skipped.

Change-Id: I61d76ef3618a6bc7e3228fa7ee8bf32dafaec53d
Reviewed-on: https://gerrit.chromium.org/gerrit/60953
Reviewed-by: Alex Miller <milleral@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
Tested-by: Christopher Wiley <wiley@chromium.org>
3 files changed