Test: force merge index in the end of IndexStatsTests.throttleStats

This works around slow IO (fsync) causing the test-framework cleanup
to timeout at 30 seconds when trying to delete the index.

Closes #8528
[TEST] Register data.path for all nodes on close in InternalTestCluster

We need to register those data paths otherwise we might miss path that
need to get cleaned when using local gatway etc. which can otherwise
cause imports of dangeling indices.