PulsarIOTest.testReadFromSimpleTopic is very flaky

PulsarIOTest uses shared client across all tests [1], which is a race conditions when multiple tests run in parallel.  This manifests as frequent test failures.

[1] https://github.com/apache/beam/blob/master/sdks/java/io/pulsar/src/test/java/org/apache/beam/sdk/io/pulsar/PulsarIOTest.java#L62