[gdb/testsuite, tclint] Fix unrecognized argument in gdb.trace/mi-traceframe-changed.exp

[gdb/testsuite, tclint] Fix unrecognized argument in gdb.trace/mi-traceframe-changed.exp

When running tclint on gdb.trace/mi-traceframe-changed.exp, I get:
...
$ tclint gdb.trace/mi-traceframe-changed.exp
$exp:94:1: unrecognized argument for append: -1 [command-args]
$exp:95:1: unrecognized argument for append: -1 [command-args]
...
for these lines:
...
   94  append testfile -1
   95  append binfile -1
...

This seems harmless to me, but since tclint complains, fix this by quoting the
-1 arguments.

Tested by rerunning tclint.

Approved-By: Simon Marchi <simon.marchi@efficios.com>
PR testsuite/33403
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33403