Shenandoah: Fix JFR test failures

With the recent Shenandoah backport to jdk11u, we decided to optionally split metadata.xml so that Shenandoah can provide its own metadata-shenandoah.xml in a separate file. This avoids generating code for Shenandoah when Shenandoah is not present in the build.

However, the metadata.xml is also read at run-time, but metadata-shenandoah.xml is not present at run-time, nor would it be read with the current implementation of MetadataHandler.java. This makes the test jdk/jfr/tool/TestPrintJSON.java fail.