Preparing to switch away from the deprecated test runner

A lot of ravenwood tests (and other tests too) use the deprecated
test runner androidx.test.runner.AndroidJUnit4, but it actually has
problems. We should switch them to
androidx.test.ext.junit.runners.AndroidJUnit4.

However, simply updating all tests could be problematic because:
- we need to update the build rules and add "androidx.test.ext.junit"
as a static dependency, which is probably safe but it could
cause class conflicts.(?)
- More importantly, the subtle difference between the test runner
could make the device side test fail.

So I'm going to do it in two steps:

- Step 1 -- this CL:
Add an optional validator to RavenwoodTest, which can be enabled
with RAVENWOOD_OPTIONAL_VALIDATION=1.

This makes a test fail if it's using the deprecated one.

- Added a test for this

- Added a script to replace the test runner.

Test: run-ravenwood-tests
Test: RAVENWOOD_OPTIONAL_VALIDATION=1 atest RavenwoodCoreTest
Bug: 317131861

Change-Id: I5762bd097c3a529ae6e14460d9e326ed5c6086b4