Enable tests in build

Created by: Zaharid

This implements some but not all of the points in #95 (closed). In order to be able to merge it, we require a low precision theory that we can use it tests. Currently, it predictably fails with:

  [utils] error: Could not open /home/zah/anaconda3/conda-bld/
  nnpdf_1519818938025/
  _test_env_placehold_placehold_placehold_placehold_placehold_placehold_placeh-
  old_placehold_placehold_placehold_placehold_placehold_placehold_placehold_pl-
  acehold_placehold_placehold_placehold_placehold_place/share/NNPDF/data/
  theory_53/fastkernel/FK_NMC.dat

It is also missing enabling coverage reports, but that's out of scope here.