Python extensions accept explicit target name
Add add_pytest function

Adds any numbers of python tests for the py.test framework.
These tests are not installed. That would be the job of the add_python_module
function.
:bug: when installing python extension

Bug appears when location variable is empty. It is empty when the extension is
not contained within a package.
add_python_module accepts configurable files

All *.in.* are passed through configure_file with @ONLY.
Add function so ctest can work with py.test