tests: Disable pylint preexec-fn warning

Related: rhbz#1785154

src/pylorax/executils.py

@@ -115,6 +115,7 @@ def startProgram(argv, root='/', stdin=None, stdout=subprocess.PIPE, stderr=subp
     if env_add:
         env.update(env_add)
 
+    # pylint: disable=subprocess-popen-preexec-fn
     return subprocess.Popen(argv,
                             stdin=stdin,
                             stdout=stdout,