Test suite: get rid of the JRuby mess

We want to drop the dependency of JRuby from our automated test suite, and move to native Ruby.

See the blueprint.

Blueprint: https://tails.boum.org/blueprint/test_suite_getting_rid_of_the_jruby_mess/

Feature Branch: test/rjb-migration

Parent Task: #6315 (closed)

Subtasks

• #6313 (closed)
• #6399 (closed)
• #6400 (closed)

Original created by @tails on 5731 (Redmine)