[gdb/testsuite] Don't return -1 from top-level

Replace:
...
if {<cond>} {
   return -1
}
...
with:
...
if {<cond>} {
   return
}
...

See also commit 51f8decb2fc ("GDB: testsuite: TUI: Don't return -1 from
top-level (sed)").

Generated by a script written by Claude Code.

Tested on x86_64-linux and aarch64-linux.

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