Set abs_zero for adv_diff_reaction_test

closes #3730

The "phi" field in this test has a range centered around zero at each step of the transient.  Furthermore, that field reverses over time.  I noticed that it is using unusual PETSc options:

```
  petsc_options_iname  = '-pc_factor_levels -pc_factor_mat_ordering_type'
  petsc_options_value  = '20 rcm'
```

At this time it appears that normal day to day code changes have somehow caused a slightly different round-off error in this test on the Intel platform. I have verified that the diffs on Clang are in the 1e-15 range for all timesteps.  Convergence on Intel appears the same.  With that in mind, I think the best course of action would be to set an absolute floor on this test.