tests: quote $PYTHON in test-byteify-strings.t

This is needed because the default install location is "Program Files" on
Windows.

Differential Revision: https://phab.mercurial-scm.org/D7251

tests/test-byteify-strings.t

@@ -1,7 +1,7 b''
 #require py37
 
   $ byteify_strings () {
-  >   $PYTHON "$TESTDIR/../contrib/byteify-strings.py" "$@"
+  >   "$PYTHON" "$TESTDIR/../contrib/byteify-strings.py" "$@"
   > }
 
 Test version