Use the compiled extension in test

[etiennebarrie]

test_gc fails when using bundle exec because it loads JSON from stdlib. It can be seen by showing the $LOADED_FEATURES in the test, without using bundle exec. This makes sure we use code from lib and ext when running the fork.

  test_gc:																															F
================================================================================================================================================================================================================================================================================
Failure: test_gc(JSONGeneratorTest):
  pid 58791 exit 1
  | /Users/etienne/.gem/ruby/3.3.1/gems/bundler-2.5.16/lib/bundler/runtime.rb:304:in `check_for_activated_spec!': You have already activated json 2.7.1, but your Gemfile requires json 2.7.2. Since json is a default gem, you can either remove your dependency on it or try updating to a newer version of bundler that supports json as a default gem. (Gem::LoadError)
  | 	from /Users/etienne/.gem/ruby/3.3.1/gems/bundler-2.5.16/lib/bundler/runtime.rb:25:in `block in setup'
  | 	from /Users/etienne/.gem/ruby/3.3.1/gems/bundler-2.5.16/lib/bundler/spec_set.rb:191:in `each'
  | 	from /Users/etienne/.gem/ruby/3.3.1/gems/bundler-2.5.16/lib/bundler/spec_set.rb:191:in `each'
  | 	from /Users/etienne/.gem/ruby/3.3.1/gems/bundler-2.5.16/lib/bundler/runtime.rb:24:in `map'
  | 	from /Users/etienne/.gem/ruby/3.3.1/gems/bundler-2.5.16/lib/bundler/runtime.rb:24:in `setup'
  | 	from /Users/etienne/.gem/ruby/3.3.1/gems/bundler-2.5.16/lib/bundler.rb:164:in `setup'
  | 	from /Users/etienne/.gem/ruby/3.3.1/gems/bundler-2.5.16/lib/bundler/setup.rb:32:in `block in <top (required)>'
  | 	from /Users/etienne/.gem/ruby/3.3.1/gems/bundler-2.5.16/lib/bundler/ui/shell.rb:159:in `with_level'
  | 	from /Users/etienne/.gem/ruby/3.3.1/gems/bundler-2.5.16/lib/bundler/ui/shell.rb:111:in `silence'
  | 	from /Users/etienne/.gem/ruby/3.3.1/gems/bundler-2.5.16/lib/bundler/setup.rb:32:in `<top (required)>'
  | 	from -:in `require'
  .