Fixes for run-test 656-annotation-lookup-generic-jni.

- Fix test description.
- Move Main.class from `src` to `src-art, as this file uses libcore
  classes.
- Ignore "JNI_OnUnload called" as last output line, which is
  emitted on gcstress configurations.

Test: art/test/testrunner/testrunner.py -t 656-annotation-lookup-generic-jni (with and without `ANDROID_COMPILE_WITH_JACK=false`)
Bug: 38454151
Bug: 34659969
Change-Id: Ifdb59935621b1326d1504cb86e1c9cd343dd054e