Make Spec the default reporter

Mocha's [default reporter](https://mochajs.org/#reporters) has changed from Dot to Spec and I think we should follow suit. Dot may be pretty to some but is useless when it is important: when there is a failure. Timeouts during tests are useless (unless you wanna count dots and memorized test execution order). Even if all tests pass, it is very easy to not notice that some tests did not run because you cannot see their titles. The spec reporter also reports test slowness per test.