I'm assuming you're running "CptS":s project at https://github.com/CptS/solrj-classloader-leak (rather than my project at https://github.com/mjiderhamn/classloader-leak-prevention/ ). I don't think I actually ran that myself, so I cannot answer for it, but I know the Solr server must be responding for the leak to be triggered by that code.
The simplified steps detailed in the bug report, or my test case at https://github.com/mjiderhamn/classloader-leak-prevention/blob/master/classloader-leak-prevention/classloader-leak-prevention-core/src/test/java/se/jiderhamn/classloader/leak/prevention/cleanup/X509TrustManagerImplUnparseableExtensionCleanUpTest.java , should allow you to reproduce the issue however. If it helps, I can also provide a heap dump with the leak in it. (Possibly I could even write a stand alone application that fills metaspace with leaked classloaders).