Optimizing: Avoid blocked registers in gtests.

Unfortunately, one of the tests cannot work without blocked
registers with the old no-output-overlap handling without a
substantial rewrite, so we skip it when the corresponding
flag is disabled.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 438388740
Flag: EXEMPT TEST_ONLY
Change-Id: If7636e9d7e94b0387086b1f18b9bbb73d7bbdb7c