self-tests don't set the right PATH

[stanislavlevin]

With e60384c PATH is unconditionally set to os.environ.get('PATH', '').
This leads to unability to find out utils during testing:

make test
...


cd . && ./tests/magtests.py --path /usr/src/RPM/BUILD/apache2-mod_auth_gssapi-1.6.3/testsdir --so-dir=/usr/src/RPM/BUILD/apache2-mod_auth_gssapi-1.6.3/src/.libs
Traceback (most recent call last):
  File "./tests/magtests.py", line 807, in <module>
    kdcproc, kdcenv = setup_kdc(testdir, wrapenv)
  File "./tests/magtests.py", line 330, in setup_kdc
    ksetup = subprocess.Popen(["kdb5_util", "create", "-W", "-s",
  File "/usr/lib64/python3.8/subprocess.py", line 854, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib64/python3.8/subprocess.py", line 1702, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'kdb5_util'
make: Leaving directory '/usr/src/RPM/BUILD/apache2-mod_auth_gssapi-1.6.3'

$ PATH=$PATH:/usr/sbin which kdb5_util
/usr/sbin/kdb5_util