performance tests should not always run from mocha

[demmer]

I was trying to run the test coverage scripts against a subset of the tests by opting into the tests using it.only. However because the performance tests are not run as part of a describe or it block but exist inside the test directory, they were always run.

It seems better to either move the performance tests to another top-level directory besides test, or wrap them inside a mocha-style describe block.