[autotest] Don't schedule HWTests if less than 4 duts in the pool is available.

BUG=chromium:372085
TEST=unittest, test run_suite against different boards/pools in cautotest
Confirm run_suite returns warning if not enough duts available.

CQ-DEPEND=CL:198802

Change-Id: Ic2515866727a44a32869057d29e2f253cf553602
Reviewed-on: https://chromium-review.googlesource.com/199941
Reviewed-by: Dan Shi <dshi@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
4 files changed