gdb, testsuite, python: Add missing imports.

Removing the pretty printer (bound_registers.py) in the next commit
leads to failures due to a missing import of 'gdb.printing':

"AttributeError: module 'gdb' has no attribute 'printing'".

Add this import to each file requiring it, as it's not imported by the
pretty-printer anymore.

Approved-By: Andrew Burgess <aburgess@redhat.com>

gdb/python/lib/gdb/dap/varref.py

@@ -18,6 +18,7 @@ from collections import defaultdict
 from contextlib import contextmanager
 
 import gdb
+import gdb.printing
 
 from .server import client_bool_capability
 from .startup import DAPException, in_gdb_thread