Make RAVENWOOD_RUN_DISABLED_TESTS easier to understand

When the environment variable RAVENWOOD_RUN_DISABLED_TESTS is set

1. The logic for disabling/enabling tests is inverted; that is, tests
   that were originally enabled will be skipped, and tests that were
   originally disabled will run.

2. The regex pattern set in RAVENWOOD_REALLY_DISABLED can be used to
   "really" skip a test, even when RAVENWOOD_RUN_DISABLED_TESTS is
   enabled.

3. The existing behavior to "invert" the test result is removed. After
   this change, the test results will always be accurate, regardless of
   the flag used.

We also take the opportunity to remove the "private" APIs in
RavenwoodRule, and move tests that require poking into Ravenwood
internals to RavenwoodCoreTest.

Bug: 292141694
Flag: EXEMPT host side change only
Test: f/b/r/scripts/run-ravenwood-tests.sh
Change-Id: I8a8c2ee029d70e095da18e37a8487c2db7ae00dc