[gdb/testsuite] Fix gdb.server/multi-ui-errors.exp for remote target

[gdb/testsuite] Fix gdb.server/multi-ui-errors.exp for remote target

Test-case gdb.server/multi-ui-errors.exp fails for target board
remote-gdbserver-on-localhost with REMOTE_TARGET_USERNAME=remote-target:
...
(gdb) PASS: gdb.server/multi-ui-errors.exp: interact with GDB's main UI
Executing on target: kill -9 6447    (timeout = 300)
builtin_spawn [open ...]^M
XYZ1ZYX
sh: line 0: kill: (6447) - Operation not permitted
...

The problem is that the kill command:
...
remote_exec target "kill -9 $gdbserver_pid"
...
intended to kill gdbserver instead tries to kill the ssh client session in
which the gdbserver runs, and fails because it's trying as the remote target
user (remote-target on localhost) to kill a pid owned by the the build user
($USER on localhost).

Fix this by getting the gdbserver pid using the ppid trick from
server-kill.exp.

Likewise in gdb.server/server-kill-python.exp.

Tested on x86_64-linux.