tests: use the right python when running dummyssh for narrow

some plateform no longer have a `python` executable.

Differential Revision: https://phab.mercurial-scm.org/D9504

tests/narrow-library.sh

@@ -2,7 +2,7 b' cat >> $HGRCPATH <<EOF'
 [extensions]
 narrow=
 [ui]
-ssh=python "$RUNTESTDIR/dummyssh"
+ssh=$PYTHON "$RUNTESTDIR/dummyssh"
 [experimental]
 changegroup3 = True
 EOF

tests/test-narrow-widen-no-ellipsis.t

@@ -107,7 +107,7 b' added upstream revisions.'
 
   $ hg tracked --addinclude widest/f --debug
   comparing with ssh://user@dummy/master
-  running python "*dummyssh" *user@dummy* *hg -R master serve --stdio* (glob)
+  running * "*dummyssh" *user@dummy* *hg -R master serve --stdio* (glob)
   sending hello command
   sending between command
   remote: * (glob)