test: make test-extdiff resilient to /usr/bin/echo

My test machine has 'echo' in '/usb/bin/echo', #dontaskmewhy.

tests/test-extdiff.t

@@ -179,7 +179,7 b' Test pre-72a89cf86fcd backward compatibi'
 TODO
 #else
   $ hg --debug odd | grep '^running'
-  running "/bin/echo --foo='sp ace' 'sp ace' --bar='sp ace' 'sp ace'" in * (glob)
+  running "*/bin/echo --foo='sp ace' 'sp ace' --bar='sp ace' 'sp ace'" in * (glob)
 #endif
 
 #if execbit