Fix warnings and prevent Jbuilder::VERSION constant from being obliterated

[pixeltrix]

Fixes the following warnings:

• Warning about routes.rb file not existing when running tests
• Warning about method redefinition in test_helper.rb
• Warning about constant redefinition in jbuilder.rb

Some warnings remain about gem loading but they appear to be upstream issues.

Also fixes bin/test to use the correct path for the env command and adds Rails 7.2 to the Appraisals file.