test: add test for flexible edge case

[jansenk]

when a learner recieves the neccessary number of peer grades, but takes more than seven days to complete thir own peer reviews, we still reduce the number of grades used to calculate the final score, even though more are available

JIRA: https://2u-internal.atlassian.net/browse/AU-1871

This is just a test to capture the behavior we're interested in. A fix will be addressed in AU-1904 FYI: @openedx/content-aurora

[codecov[bot]]

Codecov Report

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

Project coverage is 95.04%. Comparing base (7c4c1a1) to head (1842ae9).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2191 +/- ## ======================================= Coverage 95.03% 95.04% ======================================= Files 191 191 Lines 21003 21029 +26 Branches 1896 1900 +4 ======================================= + Hits 19960 19986 +26 Misses 779 779 Partials 264 264 ``` | [Flag](https://app.codecov.io/gh/openedx/edx-ora2/pull/2191/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openedx) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/openedx/edx-ora2/pull/2191/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openedx) | `95.04% <100.00%> (+<0.01%)` | :arrow_up: | 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=openedx#carryforward-flags-in-the-pull-request-comment) to find out more.

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[nsprenkle]

@jansenk , do you have the fix ticket groomed yet? Once that is linked, I think this is good to go.