[rubygems/rubygems] Skip flaky tests on jruby

I believe these are most certainly due to a problem outside of rubygems.
I'm skipping these at least until webrick officially supports jruby
(defining "officially" as "webrick's CI passes on jruby").

https://github.com/rubygems/rubygems/commit/4fca025358

test/rubygems/test_gem_remote_fetcher.rb

@@ -1051,6 +1051,8 @@ PeIQQkFng2VVot/WAQbv3ePqWq07g1BBcwIBAg==
   end
 
   def start_ssl_server(config = {})
+    skip "starting this test server fails randomly on jruby" if Gem.java_platform?
+
     null_logger = NilLog.new
     server = WEBrick::HTTPServer.new({
       :Port => 0,