Fix failure with C++ exceptions in DAP

While working on earlier patches, I noticed that the DAP C++ exception
test had some strange results in the log.  Digging into this, I found
that while the Ada catchpoints emit a "bkptno" field in the MI result,
the C++ ones do not -- but the DAP code was relying on this.

This patch fixes the problem by changing which field is examined, and
then updates the tests to verify this.

Reviewed-by: Keith Seitz <keiths@redhat.com>