tests: clean up two wayward `python` invocations

These are also fallout from the check-code regex rewriting bug.

Differential Revision: https://phab.mercurial-scm.org/D2865

tests/test-narrow-clone-non-narrow-server.t

@@ -19,7 +19,7 b' Test attempting a narrow clone against a'
 
 Verify that narrow is advertised in the bundle2 capabilities:
   $ echo hello | hg -R . serve --stdio | \
-  >   python -c "import sys, urllib; print urllib.unquote_plus(list(sys.stdin)[1])" | grep narrow
+  >   $PYTHON -c "import sys, urllib; print urllib.unquote_plus(list(sys.stdin)[1])" | grep narrow
   narrow=v0
 
   $ cd ..

tests/test-sparse-clone.t

@@ -2,7 +2,7 b' test sparse'
 
   $ cat >> $HGRCPATH << EOF
   > [ui]
-  > ssh = python "$RUNTESTDIR/dummyssh"
+  > ssh = $PYTHON "$RUNTESTDIR/dummyssh"
   > username = nobody <[e-mail]>
   > [extensions]
   > sparse=