[autotest] make test_that able to schedule individual tests

BUG=chromium:246750
TEST=new unittest in suite_unittest, passes. Manually ran test_that with
a mix of various test / suite names in combination, and tests ran
correctly.

Change-Id: Ia7bd73fc916398d6e63a031dc6244b6fc6f4e0f8
Reviewed-on: https://gerrit.chromium.org/gerrit/58242
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Tested-by: Aviv Keshet <akeshet@chromium.org>
Commit-Queue: Aviv Keshet <akeshet@chromium.org>
3 files changed