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

With test-case gdb.server/server-kill-python.exp, I sometimes run into:
...
builtin_spawn gdb -nw -nx -q -iex set height 0 -iex set width 0 \
  -data-directory data-directory^M
kill^M
(gdb) kill^M
file server-kill-python^M
The program is not being run.^M
(gdb) ERROR: Couldn't load server-kill-python into GDB.
...

The problem is that the spawn produces a prompt, but it's not explicitly
consumed.

This is a regression since commit 0f077fcae0f ("[gdb/testsuite] Simplify
gdb.server/server-kill-python.exp").

Fix this by consuming the initial prompt.

PR testsuite/31819
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31819
Fixes: 0f077fcae0f ("[gdb/testsuite] Simplify gdb.server/server-kill-python.exp"