reduce duplication in mocha testing configuration

[thescientist13]

Type of Change

• New Feature Request
• Documentation / Website
• Improvement / Suggestion
• Bug
• Other (please clarify below)

Summary

Coming out of #292 , changes to mocha configuration where needed to handle the extra directory structure, which combined with existing options and work arounds has made the npm scripts for test and test:tdd are getting a little cumbersome.

  "test": "export BROWSERSLIST_IGNORE_OLD_DATA=true && yarn clean && nyc mocha ./packages/**/test/**/**/*.spec.js ./packages/**/test/**/**/**/*.spec.js --timeout 30000",
    "test:tdd": "export BROWSERSLIST_IGNORE_OLD_DATA=true  && yarn clean && mocha --watch ./packages/**/test/**/**/*.spec.js ./packages/**/test/**/**/**/*.spec.js --timeout 15000"

Details

Since there's also a lot of duplication, it would probably be best to create a .mocharc.js file and resue that in both scripts.