avocado.utils.datadrainer: introduce functional tests

[clebergnu]

The avocado.utils.datadrainer module currently lacks functional tests, interacting with live data "producers". We should have a better test coverage that stresses the current implementation. A minimal coverage should include:

1. Draining data from various types of sources 1.1. plain files 1.2. sockets 1.3. custom objects providing fileno()
2. Proper error handling 2.1. closing the producer while draining from it 2.2. pausing and resuming the producer