Failing Unit Test `mam4_amicphys_1gridcell_tests()`

[mjs271]

As of #345, which changed nucleation from the box model implementation to the refactored MAM4 version, the unit test mam4_amicphys_1gridcell_tests() is failing and disabled as a result.

The test fails due to an incorrect value for qgas_cur[igas_h2so4].

The cause is likely attributable to a difference in the indexing convention between the box model and MAM4, so the fix may be as simple as swapping the index that the unit test is using for h2so4.