Add regression test for instructionReference change

Yesterday I pushed a patch to fix a small oversight in the DAP code
that caused an instructionReference to be an array instead of a
string.

This patch adds a test case for that regression.  This required
exposing the TON form of the response -- something I mentioned might
be necessary when this code was changed to return just the Tcl form.

I tested this by backing out yesterday's bug fix and verifying that a
failure is seen.