Make TestNative work without -nativepath

TestNative compiles a JNI method and verifies that a native sample event can be taken. This works well when running tests using 'make test', but is a nuisance when running JFR test from the command line as you need to specify -nativepath or the test fails.

Instead of compiling a native method, we could use an already existing native method in the jdk.jfr.JVM class.