[TEST] Add test for accessing _score in scripts
[TEST] Fix test cluster naming

This commit renames `TestCluster` -> `InternalTestCluster` and
`ImmutableTestCluster` to `TestCluster` for consistency. This also
makes `ExternalTestCluster` and `InternalTestCluster` consistent
with respect to their execution environment.

Closes #6510
[DOCS] Fixed is/if typo in Api Conventions doc
[TEST] wait for ongoing recoveries to finish. Flush fails on shards otherwise.