test_that: Use default board saved by setup_board if none is provided.

BUG=chromium:264879
TEST=Ran test_that without the --board flag to verify default board is detected.

Change-Id: I2703e7ddf64fb8a9fc80cf90b933b3ec03ecd685
Reviewed-on: https://gerrit.chromium.org/gerrit/63493
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
Commit-Queue: Prathmesh Prabhu <pprabhu@chromium.org>
1 file changed