Similar to --max-steps-performed, don't do any calibration and
just run flat out for a fixed amount of time.

Also rename --test-time to --max-trial-time to reflect what the
parameter actually does, limiting each trial to a ceiling.

Also remove a redundant check in doRunLoop.

Test: angle_perftests --fixed-test-time 20
Test: angle_perftests --max-trial-time 2
Bug: angleproject:5935
Change-Id: I854799a734cd6c019399f5a1444b4ef5f659ddf4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2878937Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Commit-Queue: Cody Northrop <cnorthrop@google.com>