Update TESTING.md shared test instructions

[maureensturgeon]

Update instructions for running shared unit tests from ruby -Itest ./test/path/to/your/test.rb to bundle exec ruby -Itest ./test/path/to/your/test.rb

PR Checklist:

• Tests provide adequate coverage
• Privacy and Security impacts have been assessed
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked

[maddiedierker]

thank you, maureen!