Fix classpath issues in ClasspathJrt and model tests

[stephan-herrmann]

• Ensure ClasspathJrt is initialized to fix VisitorTest failing since I20241015-1820 eclipse.jdt.ui#1722
• Fix test's getJCL15PlusLibraryIfNeeded()

[stephan-herrmann]

I merged this already to fix eclipse-jdt/eclipse.jdt.ui#1722 for the next I-Build, but still I'd like to get a word from people having worked in this area, like @jukzi @jarthana

ClasspathJrt and friends don't seem to be usable before initialize() is called, but I could not see what kind of strategy was intended.

Is initialization supposed to happen on demand or eagerly? Neither strategy seems to be implemented consistently.

Backing up one step: is there reason to create a ClasspathJrt without initialization? Sure, instantiation is faster without, but is instantiation without initialization meaningful?

[jarthana]

I definitely remember that initialize() should happen before everything else. But then I also remember us having few issues in this area and may have had some back-forth changes.

[stephan-herrmann]

Fix test's getJCL15PlusLibraryIfNeeded()

For posterity a word on why I made this change: when running model tests with -Dcompliance=23 or any setting that doesn't include 1.8, then ClasspathTest.testBug576735a would throw NPE because in this configuration getJCL15PlusLibraryIfNeeded() returned null, simply because the requested "1.8" is not in the set of compliances specified via the system property. I'm quite sure that when explicitly requesting the JCL lib for a specific compliance, then that library should be provided (and not hardcoded "1.8"), no matter what system property was passed in.