[ruby/irb] Fix usage of tracer gem and add tests

(https://github.com/ruby/irb/pull/857)

The new tests are skipped when ruby below 3.1, as it was a default gem on it, and in a version we do not support.

This also move definition of `use_tracer` to module Context instead of monkey patch.

08834fbd5f

lib/irb/extend-command.rb

@@ -317,7 +317,6 @@ module IRB # :nodoc:
 
     @EXTEND_COMMANDS = [
       [:eval_history=, "ext/eval_history.rb"],
-      [:use_tracer=, "ext/tracer.rb"],
       [:use_loader=, "ext/use-loader.rb"],
     ]