add remote-test capability

To execute tests on a remote host (for instance, because you're cross-compiling),
simply do:

make remote-test=true remote-test-host=<host_to_test_on> test

You can set several variables to control the functionality of remote-test.
See them below, along with their default values:

remote-test-host = localhost # host to ssh to
remote-test-port = 22
remote-test-user = ${USER} # user to execute tests as
remote-test-dir = /tmp/avian-test-${USER} # dir to rsync build output to