Speed up JRuby tests by improving its startup time

Emulate `jruby --dev` which improves JRuby's startup time.

See
https://github.com/jruby/jruby/wiki/Improving-startup-time#use-the---dev-flag

Follow @headius' idea and give Java more memory to reduce GC time.
Merge pull request #16613 from splattael/master

Speed up JRuby tests by improving the startup time