[gdb/testsuite] Add test-case gdb.tui/long-prompt.exp

I noticed that the test-suite doesn't excercise the case in
tui_redisplay_readline that height (initially 1) is changed by this call:
...
    tui_puts_internal (w, prompt, &height);
...

Add a test-case that excercises this.

Tested on x86_64-linux.
1 file changed