Fix gdb.python/py-connection.exp with remote targets

Fix gdb.python/py-connection.exp with remote targets

After the patch to make gdb_test's question non-optional when
specified, gdb.python/py-connection.exp started failing like so:

  $ make check TESTS="gdb.python/py-connection.exp" RUNTESTFLAGS="--target_board=native-gdbserver"
  (gdb) PASS: gdb.python/py-connection.exp: info connections while the connection is still around
  disconnect^M
  Ending remote debugging.^M
  (gdb) FAIL: gdb.python/py-connection.exp: kill the inferior

The problem is that "disconnect" when debugging with the native target
asks the user whether to kill the program, while with remote targets,
it doesn't.

Fix it by explicitly killing before disconnecting.

Tested with --target_board unix, native-gdbserver, and native-extended-gdbserver.

Change-Id: Icd85015c76deb84b71894715d43853c1087eba0b