Specifying 'async mode' for testing

[Vladimir Prus]

While updating MI testsuite for async mode and other discussed changes,
I find it preferable to know if we're testing async-enabled target, or
not. In particular, because in async mode the order of some output is
different, and creating a regexp that will catch all valid output and
will not miss invalid output is a bit complex.

What's the best way to specify async mode? One approach is use a board
file, and put:

	set_board_info async 1

there, but creating board file is a nuisance. Is there any way to specify
async mode on the command line of runtest?

Thanks,
Volodya