[gdb/testsuite] Consume output asap in gdb.server/multi-ui-errors.exp

[gdb/testsuite] Consume output asap in gdb.server/multi-ui-errors.exp

With test-case gdb.server/multi-ui-errors.exp we see:
...
(gdb) PASS: multi-ui-errors.exp: main UI, prompt after gdbserver dies
continue^M
Continuing.^M
echo^M
(gdb) PASS: multi-ui-errors.exp: extra UI, prompt after gdbserver dies
...

The continue is issued earlier in the test-case, but the output has not been
consumed, which makes it show up much later.

Consume the continue output asap, to make it clear when the continue is issued:
...
(gdb) PASS: gdb.server/multi-ui-errors.exp: connect to gdbserver
continue^M
Continuing.^M
PASS: gdb.server/multi-ui-errors.exp: continue - extra UI
...

Tested on x86_64-linux.