Fix is_remote check in gdb.base/remote.exp

1. Otherwise, when the native-gdbserver board stops setting is_remote,
   this test would stop running there.

2. Makes the test run with --target_board=native-extended-gdbserver
   too.

gdb/testsuite/ChangeLog:
2017-10-13  Pedro Alves  <palves@redhat.com>

	* gdb.base/remote.exp: Check gdb_protocol instead of is_remote.
	(top level): Add comment.