make path to nosetests executable configurable

this is to make use python2 on systems when `nosetests` points to python3 or may be missing at all being unsuffixed:

    $ LC_ALL=C ls -l /usr/bin/nosetests-*
    -rwxr-xr-x 1 root root 304 Nov 14 23:22 /usr/bin/nosetests-2.7
    -rwxr-xr-x 1 root root 305 Nov 14 23:22 /usr/bin/nosetests-3.3
    $ make NOSETEST=nosetests-2.7 tests