Make eslint rule testing use jest (not mocha)

[zpao]

using npm@3, mocha is not necessarily at node_modules/eslint-tester/node_modules/mocha/bin/mocha (thanks to hoisting). (eg it's actually at node_modules/mocha/bin/mocha for me)

[bgw]

We've found out that it's possible to use eslint-tester with jest, so we should just do that.