[gdb/testsuite, Tcl 9.0] Namespace fixes

A few test-cases wrap code in namespaces.

Since Tcl 9.0, globals are no longer directly available inside namespaces.

Fix this by redeclaring using upvar.

Also, referring to variables in namespaces needs to be done using fully
qualified names.  Fix this by adding a :: prefix.

Approved-By: Tom Tromey <tom@tromey.com>

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33948