[gdb/testsuite] Fix timeout in gdb.tui/completion.exp

[gdb/testsuite] Fix timeout in gdb.tui/completion.exp

With test-case gdb.tui/completion.exp, we run into:
...
WARNING: timeout in accept_gdb_output
PASS: gdb.tui/completion.exp: check focus completions
...

The timeout happens in this command:
...
Term::command "layout src"
...
which waits for:
- "(gdb) layout src", and then
- "(gdb) ".

Because the "layout src" command enables the TUI there's just a prompt.

Fix this by using Term::command_no_prompt_prefix.

Tested on x86_64-linux.