Benchmark tests should not run on the website

[jmrunkle]

This requires changes both in the test code as well as in the actual test runner, but the simplest option is to skip tests using the short flag.

I will be creating another issue in the go test runner repo to make the relevant change for the test runner itself so that it sets the -short flag when running go test.

By using the short flag, CLI students that simply run go test will automatically see the benchmarks, but students in the UI will not be weighed down by them.

[ErikSchierboom]

Closed by exercism/go-test-runner#37