When a plug-in requires the JUnit bundle junit-jupiter-api,
if the target platform contains both JUnit 5 and JUnit 6,
RequiredPluginsClasspathContainer will put JUnit 6 on the JDT classpath.

This is problematic, since a plug-in cannot then specify JUnit 5,
even when constraining the version of junit-jupiter-api.

This change checks what version of junit-jupiter-api is required,
then adds only that version of JUnit bundles to the container.

Fixes: #2007