gdb/testsuite/dap: pass around dicts instead of TON objects

gdb/testsuite/dap: pass around dicts instead of TON objects

The DAP helper functions generally return TON objects.  However, callers
almost all immediately use ton::2dict to convert them to dicts, to
access their contents.  This commits makes things a bit simpler for them
by having function return dicts directly instead.

The downside is that the TON objects contain type information.  For
instance, a "2" in a TCL dict could have been the integer 2 or the
string "2" in JSON.  By converting to TCL dicts, we lose that
information.  If some tests specifically want to check the types of some
fields, I think we can add intermediary functions that return TON
objects, without having to complicate other callers who don't care.

Change-Id: I2ca47bea355bf459090bae8680c6a917350b5c3f