tests: test for dispatch on [defaults]: more clearly differing output

Using '-r null' instead of '-v' as the overriden command default.
The latter did not have any effect on output, thus not giving much
indication on whether the modified defaults were really in use or not.

tests/test-dispatch

@@ -15,7 +15,7 b" echo '% [defaults]'"
 hg cat a
 cat >> $HGRCPATH <<EOF
 [defaults]
-cat = -v
+cat = -r null
 EOF
 hg cat a
 

tests/test-dispatch.out

@@ -28,6 +28,6 b' options:'
 use "hg -v help cat" to show global options
 % [defaults]
 a
-a
+a: No such file in rev 000000000000
 % no repo
 abort: There is no Mercurial repository here (.hg not found)!