Wipe repositories more often

[ywelsch]

Fixes an issue where repositories are unintentionally shared among tests (given that the repo contents is captured in a static variable on the test class, to allow "sharing" among nodes) and two tests randomly chose the same snapshot name, leading to a conflict.

Test failure for GCS: https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+7.x+release-tests/359/console
Test failure for S3: https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+7.0+matrix-java-periodic/ES_BUILD_JAVA=openjdk12,ES_RUNTIME_JAVA=java12,nodes=immutable&&linux&&docker/137/console

Relevant output:

InvalidSnapshotNameException[[gqu:s] Invalid snapshot name [s], snapshot with the same name already exists]
	at __randomizedtesting.SeedInfo.seed([F3A37A4723ACFE7F:33C41934FE630348]:0)
	at org.elasticsearch.repositories.blobstore.BlobStoreRepository.initializeSnapshot(BlobStoreRepository.java:393)
	at org.elasticsearch.snapshots.SnapshotsService$2.doRun(SnapshotsService.java:414)
	at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:758)
	at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)

[elasticmachine]

Pinging @elastic/es-distributed

[original-brownbear]

LGTM :)

[original-brownbear]

@ywelsch can you add a "closes #42519" here please? :)