ahat-tests: Use LOCAL_TEST_CONFIG to set test config

Stop building a test config for ahat, just ahat-tests.

Bug: 110982517
Bug: 113503164
Bug: 113502250
Test: . build/envsetup.sh; lunch
      m -j ahat-tests. Then,
      ./host/linux-x86/testcases/ahat-tests/ahat-tests.config
      will be generated.
      m -j ahat; No test config will be generated.

      Or  atest ahat-tests

Change-Id: Id4a9c3f5fea18d112f1495f313c02d2fdb5c1443