chore: add mocha eslint rules

[eablack]

This adopts eslint-plugin-mocha to provide some conventions for our tests (importantly to avoid future issues such those discovered in #2846.

• I changed several rules to warn instead of error in order to defer some work to later and allow some flexibility for us
• A few packages related to eslint, mocha, and chai were updated along the way
• The large number of files changes is mostly due to automatically --fixing the mocha/no-mocha-arrows rule
• I made the rules to ignore javascript test files entirely since we plan to migrate those tests anyway

[k80bowman]

Thank you for digging into this!