[gdb/testsuite] Fix gdb.python/py-completion.exp

With test-case gdb.python/py-completion.exp and target board
native-extended-gdbserver I get this warning:
...
(gdb) PASS: gdb.python/py-completion.exp: discard #2
completefilecommandcond $outputs/gdb.python/py-completion/py-completion-t^G\
  PASS: gdb.python/py-completion.exp: completefilecommandcond completion
Remote debugging from host ::1, port 53346^M
monitor exit^M
not implemented^M
(gdb) WARNING: Timed out waiting for EOF in server after monitor exit
...

Fix this by adding the missing "discard #3", such that we have instead:
...
(gdb) PASS: gdb.python/py-completion.exp: discard #2
completefilecommandcond $outputs/gdb.python/py-completion/py-completion-t^G\
  PASS: gdb.python/py-completion.exp: completefilecommandcond completion
 ^M
not implemented^M
(gdb) PASS: gdb.python/py-completion.exp: discard #3
Remote debugging from host ::1, port 36278^M
monitor exit^M
(gdb)
...

Tested on x86_64-linux.

gdb/testsuite/gdb.python/py-completion.exp

@@ -72,6 +72,8 @@ if { [readline_is_used] && ![is_remote host] } {
 	}
     }
 
+    # Discarding again
+    gdb_test " " ".*" "discard #[incr discard]"
 }
 
 # Start gdb over again to clear out current state.  This can interfere