Testrunner: Stop execution when a test fails with ART_TEST_KEEP_GOING=false

Previously, the testrunner used to wait for all the existing thread to
finish execution which doesn't actually fit the requirement.

Test: run testrunner with few failing tests
Bug: 68268743
Change-Id: I1ad1f8c6572b4eac3c4af99caf566949aa8598e4