[gdb/testsuite] Fix gdb.tui/tui-mode-switch.exp on aarch64

[gdb/testsuite] Fix gdb.tui/tui-mode-switch.exp on aarch64

On aarch64-linux, occasionally I run into these warnings:
...
PASS: gdb.tui/tui-mode-switch.exp: set style enabled off
WARNING: timeout in accept_gdb_output
PASS: gdb.tui/tui-mode-switch.exp: no boo
WARNING: timeout in accept_gdb_output
...

The first in more detail:
...
Box Dump (40 x 1) @ (0, 11):
   11 b(gdb) b
WARNING: timeout in accept_gdb_output
...

Fix this by waiting for a prompt after leaving TUI before sending "b".

Also, while we're at it generate a few more passes.

Tested on aarch64-linux.