[extension/filestorage] fix flaky compaction test (#11054)
[extension/filestorage] fix flaky compaction test (#11054)

Fixes https://github.com/open-telemetry/opentelemetry-collector-contrib/issues/11039

The fix uses parallel subtests rather than goroutines as plausibly the observed failure was due to concurrent access to testing.T
open-telemetry/opentelemetry-collector-contrib,open-telemetry/opentelemetry-collector-contrib