SharedClusterSnapshotRestoreIT fails on CI

[abeyad]

After merging #21469, SharedClusterSnapshotRestoreIT fails on CI regularly [1], though never for me locally. It might be a cleanup issue with the test. I am using this issue to track a fix.

[1] https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+master+java9-periodic/619/console

[abeyad]

This turned out to be a general issue with java9, version 143 causing test failures. @ywelsch has opened up an issue with infra to downgrade java9 back to version 139 for now.