Do not allow DAP clients to dereference "void *"

While investigating a different bug, I noticed that the DAP code would
report a "void *"-typed register as having children -- however,
requesting the children of this register would fail.

The issue here is that a plain "void *" can't be dereferenced.  This
patch changes the default visualizer to treat a "void *" as a scalar.

This adds a new test; but also arranges to examine all the returned
registers -- this was the first thing I attempted and it seemed
reasonable to have a test that double-checks that all the registers
really can be dereferenced as appropriate.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33228