Fix SearchableSnapshotsPersistentCacheIntegTests.testCacheSurviveRestart

[tlrx]

The test SearchableSnapshotsPersistentCacheIntegTests.testCacheSurviveRestart fails on CI when executing this last verification:

assertThat(persistentCache.getNumDocs(), equalTo(0L));

This assert verifies that the persistent cache does not contain any cache file documents after the index is deleted. At the time this verification is done, it is possible that the CacheService hasn't processed yet the shard deletion notification (in markShardAsEvictedInCache) and hasn't evicted yet the CacheFiles associated to the deleted shard (these evictions are executed using an AbstractRunnable in the generic thread pool).

This pull request sets a large cache synchronization interval setting so that no synchronization are likely to be executed in the middle of the test. It also busy-asserts that the persistent cache has deletions (ie, cache files evictions were processed) before synchronization the cache again and checks that there are no more documents in the persistent cache.

Closes #66278

[elasticmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[henningandersen]

LGTM.

[tlrx]

Thanks Henning