Relativize test image dependencies in microbenchmarks

JMH microbenchmarks may have dependencies on artifacts in the test image outside of the benchmarks.jar. This includes native libraries (built into $TEST_IMAGE/micro/native) and may soon include other test libraries like wb.jar (built into $TEST_IMAGE/lib-test/)

By moving execution to the test image root (currently we run out of jdk/make) we can make do with a relative path, which means each benchmark can supply a constant flag themselves rather than have the test runner provide one for you. And needing to be explicit about external dependencies is good, I think.

Taken together this makes the benchmarks.jar more self-contained and simpler to run: all you need is to run `java -jar micro/benchmarks.jar` from the test image root.