test: make test-extdiff resilient to */gnubin/echo

My Mac test machine has 'echo' in '/opt/local/libexec/gnubin/echo' since, well,
GNU is not BSD.

Also, I feel it need to be said about using regexes:

Some people, when confronted with a problem, think "I know, I'll use regular
expressions." Now they have two problems.

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 "*/echo --foo='sp ace' 'sp ace' --bar='sp ace' 'sp ace'" in * (glob)
 #endif
 
 Empty argument must be quoted