No PBR_TEST_REQUIREMENTS_FILES environment variable

[Florian Wilhelm]

It is possible to define own requirement files with the help of the environment variable REQUIREMENTS_FILES (which seems to be an undocumented feature btw). With this is is possible to have non-standard file names and locations for requirements.txt.
But it is not possible to have a non-standard test-requirements.txt file since there is no environment variable PBR_TEST_REQUIREMENTS_FILES used in packaging.py.