[PP-1] Share caches between kernel test methods

Works, but fails because it's working.

Resolved the test failures:

1. In KernelTestBase::enableModules() we need to clear the plugin caches to match ModuleInstaller::install() - this is because the caches can be populated before the ::enableModules() call which generally they wouldn't be in kernel tests before this change. There's a possibility there's some manual cache clears in existing tests that could be dropped but didn't look yet.

2. We need to exclude cache tables from the shared database state validation because they're ephemeral e.g. we allow those to be created and not cleaned up in a specific test method since they're created on-demand.

The plugin issue is making me wonder if we should forbid enabling a module during a test method when the environment is shared.

Tests that do this then have three options:

1. Add the module to static::$modules

2. Split the method(s) out to a separate test

(both of these will also be a performance improvement since it means less total container rebuilds)

3. Don't share the environment (can be combined with #2

@plach could we move the check to ::tearDown() maybe? (or am I missing something and it would already cover all cases?). Also what do you think about splitting that out to its own issue?

Thanks. Postponing this on that one #3631699: Prevent module list changes outside of setup when sharing the test environment.