Clarify which tests to run in the Makefile

If you put your Virtualenv in the same folder as Requests (as I
usually do), py.test will pick up any test_* files inside the
virtualenv and attempt to run them too, leading to the following error:
https://gist.github.com/kevinburke/1aa2b07e01de3a7daa15

Instead specify the test file we would like to run in the Makefile.
clarify comment
Merge pull request #2174 from kevinburke/test-clarification

Clarify which tests to run in the Makefile