[test] improve assertEquals failure output

Currently assertEquals has in the failure case sometimes confusing output like :

java.lang.RuntimeException: VM output should contain exactly one RTM locking statistics entry for method compiler.rtm.locking.TestRTMTotalCountIncrRate$Test::lock: expected 0 to equal 1
       at jdk.test.lib.Asserts.fail(Asserts.java:634)
       at jdk.test.lib.Asserts.assertEquals(Asserts.java:205)

(I don't think we really expected that for some reason 0 equals 1)
This should be improved.