Fix SearchableSnapshotDirectoryTests.testIndexSearcher()

[tlrx]

The test SearchableSnapshotDirectoryTests.testIndexSearcher() sometimes fails with the error:

java.lang.AssertionError: current thread [Thread[LuceneTestCase-1-thread-2,5,TGRP-SearchableSnapshotDirectoryTests]] may not read [...]
    at org.elasticsearch.index.store.BaseSearchableSnapshotIndexInput.assertCurrentThreadMayAccessBlobStore(BaseSearchableSnapshotIndexInput.java:148)
    at org.elasticsearch.index.store.direct.DirectBlobContainerIndexInput.openBlobStream(DirectBlobContainerIndexInput.java:343)
    at org.elasticsearch.index.store.direct.DirectBlobContainerIndexInput.readInternalBytes(DirectBlobContainerIndexInput.java:133)
    at org.elasticsearch.index.store.direct.DirectBlobContainerIndexInput.readInternal(DirectBlobContainerIndexInput.java:105)

The method assertCurrentThreadMayAccessBlobStore() correctly checks that the thread that reads blobs is a thread created by the test framework and has the TEST- prefix in its name.

But testIndexSearcher() is slightly different from other searchable snapshots tests as it is the only one that directly opens a new IndexSearcher using LuceneTestCase's newSearcher() method which randomly opens the searcher using a testing thread pool that has a different prefix.

This commit simply adds the LuceneTestCase- prefix for testing thread.

Closes #56233

[elasticmachine]

Pinging @elastic/es-distributed (:Distributed/Snapshot/Restore)

[DaveCTurner]

LGTM; this is currently only labelled v8.0.0, should it be back ported to 7.8 and 7.x too?

[tlrx]

Thanks David