tests: use ls instead of find, all files are in the same directory

In this case find has no advantage compared to ls. Descending into directories
is unnecessary, because there are none.

tests/test-gendoc.t

@@ -3,7 +3,7 b' Test document extraction'
   $ "$TESTDIR/hghave" docutils || exit 80
   $ HGENCODING=UTF-8
   $ export HGENCODING
-  $ { echo C; find "$TESTDIR/../i18n" -name "*.po" | sort; } | while read PO; do
+  $ { echo C; ls "$TESTDIR/../i18n"/*.po | sort; } | while read PO; do
   >     LOCALE=`basename "$PO" .po`
   >     echo
   >     echo "% extracting documentation from $LOCALE"