[gdb/testsuite] Fix gdbserver pid in gdb.server/server-kill-python.exp

[gdb/testsuite] Fix gdbserver pid in gdb.server/server-kill-python.exp

The commit ed32754a8c7 ("[gdb/testsuite] Fix gdb.server/multi-ui-errors.exp for
remote target") intended to addresss the problem that this command:
...
set gdbserver_pid [exp_pid -i $server_spawn_id]
...
does not return the pid of the gdbserver for remote target, but rather the one
of the ssh client session.

To fix this, it added another way of getting the gdbserver_pid.

For the trivial case of non-remote target, the PID found by either method
should be identical, but if we compare those by adding
"puts [exec ps -p $gdbserver_pid]" we get:
...
  PID TTY          TIME CMD
31711 pts/8    00:00:00 gdbserver
  PID TTY          TIME CMD
31718 pts/8    00:00:00 server-kill-pyt
...

The problem is that while the gdbserver PID is supposed to be read from the
result of "gdb.execute ('p server_pid')" in the python script, instead it's
taken from:
...
Process server-kill-python created; pid = 31718^M
...

Fix this by moving the printing of the gdbserver PID out of the python script.

Also double-check the two methods against each other, in the cases that they
should match.

Tested on x86_64-linux.

PR testsuite/31633
https://sourceware.org/bugzilla/show_bug.cgi?id=31633