chore: Write unit test for differential expression computation

[prathapsridharan]

• Add a unit/regression test reading off a real memento estimator cube
• See memento epic ticket #948 for details

[codecov[bot]]

Codecov Report

Attention: 74 lines in your changes are missing coverage. Please review.

Comparison is base (bcb2372) 86.17% compared to head (44c09e5) 85.33%. Report is 84 commits behind head on atol/memento/epic.

Files	Patch %	Lines
...e_census/experimental/diffexp/memento/diff_expr.py	48.57%	72 Missing :warning:
.../cellxgene_census_builder/build_soma/build_soma.py	83.33%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## atol/memento/epic #976 +/- ## ===================================================== - Coverage 86.17% 85.33% -0.85% ===================================================== Files 74 76 +2 Lines 5708 5885 +177 ===================================================== + Hits 4919 5022 +103 - Misses 789 863 +74 ``` | [Flag](https://app.codecov.io/gh/chanzuckerberg/cellxgene-census/pull/976/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=chanzuckerberg) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/chanzuckerberg/cellxgene-census/pull/976/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=chanzuckerberg) | `85.33% <59.34%> (-0.85%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=chanzuckerberg#carryforward-flags-in-the-pull-request-comment) to find out more.

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