test: Test `cluster` subcommands.

Signed-off-by: Blaž Hrastnik <blaz.hrast@gmail.com>
test: Add a test for `ps`

Signed-off-by: Blaž Hrastnik <blaz.hrast@gmail.com>
test: Test `flynn route add tcp`.

Signed-off-by: Blaž Hrastnik <blaz.hrast@gmail.com>