Use GMT time zone in faketime dates

This ensures that the interpretation of date strings isn't subject to
the test system's time zone.

dnf-behave-tests/dnf/steps/cmd.py

@@ -63,7 +63,7 @@ def faketime(context, direction, when):
         return
     else:
         time = when
-    context.faketime = "faketime '%s' " % time
+    context.faketime = "faketime '%s GMT' " % time
 
 
 @behave.step("today is {when}")