Unit tests for propagator

[clux]

Adds a small fake dependency tree that looks like this:

prop-base
├─┬ prop-mid-1  (1-alpine) (2017-11-27 23:27:57)
│ └── prop-leaf (1-alpine) (2017-11-27 23:27:03)
└─┬ prop-mid-2  (1-alpine) (2017-11-27 23:28:34)
│ └── prop-leaf (1-alpine) (2017-11-27 23:27:03)

then tests that the propagation looks like:

Assuming prop-leaf has been updated:
Stage 1:
- update [prop-leaf] in prop-mid-1
- update [prop-leaf] in prop-mid-2
Stage 2:
- update [prop-mid-1, prop-mid-2] in prop-base

[clux]

Should also cover lal status better than it currently is now that we have to recurse.

[coveralls]

Coverage increased (+6.0%) to 61.16% when pulling f2fe916 on prop-tests into f982fc1 on master.