[gdb/testsuite] Fix gdb.threads/step-bg-decr-pc-switch-thread.exp for native-gdbserver

With test-case gdb.threads/step-bg-decr-pc-switch-thread.exp and target board
native-gdbserver, I run into:
...
(gdb) UNSUPPORTED: gdb.threads/step-bg-decr-pc-switch-thread.exp: \
  switch to main thread
Remote debugging from host ::1, port 43914^M
monitor exit^M
Cannot execute this command while the target is running.^M
Use the "interrupt" command to stop the target^M
and then try again.^M
(gdb) WARNING: Timed out waiting for EOF in server after monitor exit
...

Fix this by following the advice and issuing an interrupt command, allowing
the following monitor exit command to succeed.

Tested on x86_64-linux.