Only create parallel test databases on schema changes

[jhawthorn]

This introduces a new key in the ar_internal_metadata table, :schema_sha1, which tracks the SHA1 of the schema file being loaded into the database.

This can be used to keep a test database exactly in sync with a schema file. This is similar to the maintain_test_schema functionality non-parallel tests currently use, but is more accurate (migrations can be changed or rolled back). I plan to update maintain_test_schema to use this instead in a future PR.

This adjusts parallelization test databases hook to only reload the schema when it has changed, and to no longer drop the database at exit.

Refs #36807

[jhawthorn]

Closing for #36873