git: add /dev to test/.gitignore

The /dev directory is also created by zdtm when running ns/ enabled tests.
Add it to the list, together with entries such as /bin and /lib.
Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/.gitignore

 /lib
 /lib64
 /bin
+/dev
 /dump
 /*.log