Fix flaky test SparkRTest

08:26:57,884 INFO org.apache.zeppelin.rest.AbstractTestRestApi:239 - Test Zeppelin terminated.
Tests run: 6, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 168.646 sec <<< FAILURE! - in org.apache.zeppelin.rest.ZeppelinSparkClusterTest
sparkRTest(org.apache.zeppelin.rest.ZeppelinSparkClusterTest) Time elapsed: 11.03 sec <<< FAILURE!
java.lang.AssertionError: expected:<FINISHED> but was:<ERROR>
at org.junit.Assert.fail(Assert.java:88)
at org.junit.Assert.failNotEquals(Assert.java:743)
at org.junit.Assert.assertEquals(Assert.java:118)
at org.junit.Assert.assertEquals(Assert.java:144)
at org.apache.zeppelin.rest.ZeppelinSparkClusterTest.sparkRTest(ZeppelinSparkClusterTest.java:104)