Enable parallel tests (#1231)
Enable parallel tests (#1231)

Testing:

Reduced the test time from 15 seconds to 12 seconds on my mac.

./gradlew azkaban-common:cleanTest azkaban-common:test

I expect the reduction will be more significant on travis.

See
http://mrhaki.blogspot.com/2010/11/gradle-goodness-running-tests-in.html
https://docs.gradle.org/current/dsl/org.gradle.api.tasks.testing.Test.html
https://docs.gradle.org/current/userguide/java_plugin.html#sec:test_execution
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