Secretkeeper: improve VTS tests

- Look for either /default or /nonsecure instances.
- Only run a `deleteAll()` test on a /nonsecure instance.
- Delete IDs on client drop to ensure a consistent state.
- Use a config that ensures tests are run single-threaded, to ensure a
  consistent state.
- Shift to using types not raw arrays.
- Add a macro for common test setup.
- Add helper methods to reduce the amount of copy-pasta in the VTS
  tests.
- Use these helpers to create some additional tests.

Test: VtsSecretkeeperTargetTest
Bug: 291224769
Change-Id: Icff0bba1bcdd66b18398ed4b64ebd2c6bc0de7e7