Renaming test files according to their language (#3407)

test/tw-1637.test.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+. bash_tap_tw.sh
+
+# This problems only appears for "modify", not for "add"
+# The 'proj:mod' is interpreted as 'proj:modified', then DOM lookup substitutes
+# the 'modified' date.
+task add a
+task 1 mod proj:mod
+task 1 _project | grep mod