One job is for all tests except the `performance` tests, while the
other job is for only the `performance` tests.

The idea is to decrease the total time it takes for all CI jobs to
complete in most cases, by splitting the long-running (currently
usually about 13 minute) Cygwin job into two less-long jobs.

In the "Test with pytest" step of the Cygwin test jobs.

This is to distinguish the newly split jobs from each other more
clearly when glancing at their steps for a run.

This removes an unnecessary trailing slash that I had not used
consistently anyway.