Add an example jmh microbenchmark to perf test group

As has always been in plan (see [perf/README.md](https://github.com/AdoptOpenJDK/openjdk-tests/tree/master/perf#adoptopenjdk-performance-testing)), we want to add some example microbenchmarks from the ones available in [jmh-jdk-microbenchmarks](https://hg.openjdk.java.net/code-tools/jmh-jdk-microbenchmarks/).

Can use a jar file from [mvn](https://mvnrepository.com/artifact/org.openjdk.jmh/jmh-core/1.23) or some other known location