Instead of using rand(), use a fixed seed RNG. This can ensure runs
use the same draw call counts no matter the time of day.

Note: there may have been a setting somewhere that was seeding the RNG
because I wasn't seeing variation between test runs locally. This
change makes it explicit.

Bug: angleproject:4029
Change-Id: Ic51bf2a6779da3b96a3357eed99c0f7fa9d81648
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2042210Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>