[gdb/testsuite] Fix gdb.server/multi-ui-errors.exp with local-remote-host-notty

[gdb/testsuite] Fix gdb.server/multi-ui-errors.exp with local-remote-host-notty

With test-case gdb.server/multi-ui-errors.exp and host board
local-remote-host-notty, I run into:
...
(gdb) PASS: gdb.server/multi-ui-errors.exp: interact with GDB's main UI
Executing on target: kill -9 29666    (timeout = 300)
builtin_spawn -ignore SIGHUP kill -9 29666^M
echo^M
Remote connection closed^M
(gdb) (gdb) FAIL: gdb.server/multi-ui-errors.exp: \
  main UI, prompt after gdbserver dies (timeout)
...

In contrast, with local-remote-host (so, everything the same but editing off):
...
(gdb) PASS: gdb.server/multi-ui-errors.exp: interact with GDB's main UI
Executing on target: kill -9 31245    (timeout = 300)
builtin_spawn -ignore SIGHUP kill -9 31245^M
Remote connection closed^M
(gdb) echo^M
(gdb) PASS: gdb.server/multi-ui-errors.exp: main UI, prompt after gdbserver dies
...

The test-case issues a kill, which results in a "Remote connection closed"
message and a prompt.

The problem is that the prompt is not consumed, so the subsequent echo may be
issued before that prompt, which causes a mismatch when matching the result
of the echo.

Fix this by consuming the "Remote connection closed" message and prompt.

Tested on x86_64-linux.