Set the AT_SPI_CLIENT variable in the tests

This ensures the test itself is not recognized as a client
by the registry. I'm not sure how that happens since we are
not loading gtk, but it does. Not being a real gtk client with
a mainloop we wasn't answering dbus calls and causing the registry
to hang and fail.
Merge pull request #5 from dnarvaez/atspi

Set the AT_SPI_CLIENT variable in the tests