PipRequirement uses the package rather than the executable `pip`.
Executable `pip` may not exist, such as a Debian Unstable system
that have only Python 3 installed and only executable `pip3`.

pip should already exist on any system that is attempting to run
the tests.  Codify this by copying the pip test dependency
from coala's `test-requirements.txt`.

Fixes coala/package_manager#57