Fix random failure on CanCleanupConnectionLeakTest.openAndKill

java.lang.AssertionError: Caught an unkown error from session holder
	at org.nuxeo.ecm.core.management.jtajca.CanCleanupConnectionLeakTest.openAndKill(CanCleanupConnectionLeakTest.java:145)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
...
Caused by: org.junit.ComparisonFailure: expected:<[tru]e> but was:<[fals]e>
	at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
	at org.nuxeo.ecm.core.management.jtajca.CanCleanupConnectionLeakTest$OpenTask.work(CanCleanupConnectionLeakTest.java:108)
	at org.nuxeo.ecm.core.management.jtajca.CanCleanupConnectionLeakTest$Task.run(Can