Flaky-test: org.apache.distributedlog.TestTruncate

`mvn test -f stream/distributedlog/core/pom.xml`
```
[ERROR] Tests run: 7, Failures: 0, Errors: 3, Skipped: 0, Time elapsed: 112.456 s <<< FAILURE! - in org.apache.distributedlog.TestTruncate
[ERROR] org.apache.distributedlog.TestTruncate.testTruncation Time elapsed: 20.812 s <<< ERROR!
org.apache.distributedlog.exceptions.BKTransmitException: Failed to write to bookkeeper; Error is (-6) Not enough non-faulty bookies available : -6
```

The tests flake (sometimes fail with error above, sometime VM crashes) on my mac but pass if I run them one by one:
```
mvn test -f stream/distributedlog/core/pom.xml -Dtest=org.apache.distributedlog.TestTruncate#testPurgeLogs
mvn test -f stream/distributedlog/core/pom.xml -Dtest=org.apache.distributedlog.TestTruncate#testTruncation
mvn test -f stream/distributedlog/core/pom.xml -Dtest=org.apache.distributedlog.TestTruncate#testExplicitTruncation
mvn test -f stream/distributedlog/core/pom.xml -Dtest=org.apache.distributedlog.TestTruncate#testOnlyPurgeSegmentsBeforeNoneFullyTruncatedSegment
mvn test -f stream/distributedlog/core/pom.xml -Dtest=org.apache.distributedlog.TestTruncate#testPartiallyTruncateTruncatedSegments
```

probably there is some race in test setup/teardown

Start by running `mvn test -f stream/distributedlog/core/pom.xml` and compare it with the individual `TestTruncate` methods listed in the issue. Read `org.apache.distributedlog.TestTruncate` with attention to test setup and teardown, then investigate the insufficient-bookies error and VM crashes; done means the full test class runs reliably without these failures.

Written by the indexing model from the issue text.