Add another subtype test

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (ddccd34) 91.98% compared to head (5da111f) 92.11%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1068 +/- ## ========================================== + Coverage 91.98% 92.11% +0.13% ========================================== Files 93 93 Lines 9903 9903 Branches 2319 2319 ========================================== + Hits 9109 9122 +13 + Misses 794 781 -13 ``` | [Files](https://app.codecov.io/gh/johnynek/bosatsu/pull/1068?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=P.+Oscar+Boykin) | Coverage Δ | | |---|---|---| | [.../src/main/scala/org/bykn/bosatsu/rankn/Infer.scala](https://app.codecov.io/gh/johnynek/bosatsu/pull/1068?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=P.+Oscar+Boykin#diff-Y29yZS9zcmMvbWFpbi9zY2FsYS9vcmcvYnlrbi9ib3NhdHN1L3JhbmtuL0luZmVyLnNjYWxh) | `96.58% <100.00%> (ø)` | | ... and [7 files with indirect coverage changes](https://app.codecov.io/gh/johnynek/bosatsu/pull/1068/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=P.+Oscar+Boykin)

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