tests: Test that send overflow doesn't abort

The new display test runs a client that makes a very large number of
trivial requests. After responding to initial setup requests, the server
is paused, letting the trivial requests fill up the Unix socket buffer,
making further writes to the socket fail. The test then checks that the
client sets an appropriate error code, and does not abort or crash.

Signed-off-by: Manuel Stoeckl <code@mstoeckl.com>