ContextHub VTS: Only enable test mode when needed

Test mode is only needed for the following scenarios:

1. Testing test mode itself.
2. Testing the endpoint discovery APIs on cuttlefish.

Since all other tests do not involve nanoapps, we do not
enable test mode to save execution time.

Bug: 379342519
Change-Id: I8f7d7bfb04fbb0856665bc8470dac7e9bbd3fe34
Flag: TEST_ONLY
Test: atest VtsAidlHalContextHubTargetTest