gc/g1/TestGCLogMessages.java test should handle non-JFR configs

JDK-8273147 introduced a more thorough GC messages test, which includes JFR. But it is not guaranteed that JFR is actually in the build, like in the case of Zero:

$ CONF=linux-x86_64-zero-fastdebug make exploded-test TEST=gc/g1/TestGCLogMessages.java

java.lang.RuntimeException: '\[debug.*Weak JFR Old Object Samples' missing from stdout/stderr

at jdk.test.lib.process.OutputAnalyzer.shouldMatch(OutputAnalyzer.java:340)
at gc.g1.TestGCLogMessages.checkMessagesAtLevel(TestGCLogMessages.java:193)
at gc.g1.TestGCLogMessages.testNormalLogs(TestGCLogMessages.java:224)
at gc.g1.TestGCLogMessages.main(TestGCLogMessages.java:199)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:568)
at com.sun.javatest.regtest.agent.MainWrapper$MainThread.run(MainWrapper.java:127)
at java.base/java.lang.Thread.run(Thread.java:833)