SourceOnlySnapshotShardTests testRestoreMinmal

[albertzaharovits]

This reproduces locally for me (on master 3914a98):

./gradlew :x-pack:plugin:core:test -Dtests.seed=69555F822415CEA1 -Dtests.class=org.elasticsearch.snapshots.SourceOnlySnapshotShardTests -Dtests.method="testRestoreMinmal" -Dtests.security.manager=true -Dtests.locale=hu-HU -Dtests.timezone=America/Argentina/Ushuaia -Dcompiler.java=10 -Druntime.java=8

   > Throwable #1: java.lang.AssertionError: expected:<15> but was:<21>
   >    at __randomizedtesting.SeedInfo.seed([69555F822415CEA1:F482086E86FD706F]:0)
   >    at org.elasticsearch.snapshots.SourceOnlySnapshotShardTests.testRestoreMinmal(SourceOnlySnapshotShardTests.java:227)
   >    at java.lang.Thread.run(Thread.java:748)

Originally, I got the failure in a PR branch: https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+pull-request/17335/console

[elasticmachine]

Pinging @elastic/es-distributed

[dnhatn]

/cc @s1monw

[dnhatn]

I've muted this test on master and 6.x.