HBASE-22153 Fix the flaky TestRestartCluster

[Apache9]

Remove the the non-null assertion, use a waitFor to wait until it is not null instead.
Increase the waiting time to make the test more stable.

[Apache-HBase]

💔 -1 overall

Vote	Subsystem	Runtime	Comment
0	reexec	236	Docker mode activated.
		_ Prechecks _
+1	hbaseanti	0	Patch does not have any anti-patterns.
+1	@author	0	The patch does not contain any @author tags.
+1	test4tests	0	The patch appears to include 1 new or modified test files.
		_ master Compile Tests _
+1	mvninstall	268	master passed
+1	compile	57	master passed
+1	checkstyle	68	master passed
+1	shadedjars	269	branch has no errors when building our shaded downstream artifacts.
+1	findbugs	147	master passed
+1	javadoc	32	master passed
		_ Patch Compile Tests _
+1	mvninstall	235	the patch passed
+1	compile	50	the patch passed
+1	javac	50	the patch passed
+1	checkstyle	67	the patch passed
+1	whitespace	0	The patch has no whitespace issues.
+1	shadedjars	257	patch has no errors when building our shaded downstream artifacts.
+1	hadoopcheck	532	Patch does not cause any errors with Hadoop 2.7.4 or 3.0.0.
+1	findbugs	155	the patch passed
+1	javadoc	33	the patch passed
		_ Other Tests _
-1	unit	15274	hbase-server in the patch failed.
+1	asflicense	32	The patch does not generate ASF License warnings.
		17787

Reason	Tests
Failed junit tests	hadoop.hbase.client.TestSnapshotTemporaryDirectoryWithRegionReplicas
	hadoop.hbase.client.TestFromClientSide
	hadoop.hbase.master.procedure.TestSCPWithReplicas
	hadoop.hbase.client.TestAdmin1
	hadoop.hbase.TestSplitMerge
	hadoop.hbase.client.TestFromClientSide3
	hadoop.hbase.client.TestFromClientSideWithCoprocessor
	hadoop.hbase.master.procedure.TestSCPWithReplicasWithoutZKCoordinated

Subsystem	Report/Notes
Docker	Client=17.05.0-ce Server=17.05.0-ce base: https://builds.apache.org/job/HBase-PreCommit-GitHub-PR/job/PR-123/1/artifact/out/Dockerfile
GITHUB PR	#123
Optional Tests	dupname asflicense javac javadoc unit findbugs shadedjars hadoopcheck hbaseanti checkstyle compile
uname	Linux f0eeb73cb1b5 4.4.0-137-generic #163-Ubuntu SMP Mon Sep 24 13:14:43 UTC 2018 x86_64 GNU/Linux
Build tool	maven
Personality	/testptch/patchprocess/precommit/personality/provided.sh
git revision	master / b2587c8
maven	version: Apache Maven 3.5.4 (1edded0938998edf8bf061f1ceb3cfdeccf443fe; 2018-06-17T18:33:14Z)
Default Java	1.8.0_181
findbugs	v3.1.11
unit	https://builds.apache.org/job/HBase-PreCommit-GitHub-PR/job/PR-123/1/artifact/out/patch-unit-hbase-server.txt
Test Results	https://builds.apache.org/job/HBase-PreCommit-GitHub-PR/job/PR-123/1/testReport/
Max. process+thread count	5177 (vs. ulimit of 10000)
modules	C: hbase-server U: hbase-server
Console output	https://builds.apache.org/job/HBase-PreCommit-GitHub-PR/job/PR-123/1/console
Powered by	Apache Yetus 0.9.0 http://yetus.apache.org

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[Apache9]

Thanks @infraio . Merged it to master first. Let's wait for a while to see if it works, and then cherry-pick to other branches.