Adjust tests that expect jmod files to be part of all JDKs

[jarthana]

          > > @jarthana Java9ElementTests.test515342b (and possibly others) explicitly tries to read _jdk_/jmods/java.base.jmod, but the temurin JDK used during production builds does not have any jmods.

https://bugs.eclipse.org/bugs/show_bug.cgi?id=515342 doesn't mention jmods, so do you by any chance remember (after more than 7 years ...) why the test was written like this?

This may have been some leftover from the effort of bug 490103 that tried to get rid of the bogus jrt-fs.jar that represents the JRE container. But in the end, we decided it depended too much on the implementation details of the JDK.

Trying to refresh memory of old discussions I came across https://bugs.eclipse.org/bugs/show_bug.cgi?id=490103#c65

Do you think we should remove the implementation for jmod support or only disable / remove those tests?

Originally posted by @stephan-herrmann in #3228 (comment)