Reduce API unit test run time on Circle

[exalate-issue-sync]

On CircleCI, the api unit tests are regularly taking longer than 12 minutes to run

https://app.circleci.com/pipelines/github/fecgov/fecfile-web-api/6936/workflows/b91de021-955d-4c32-b35d-8e4921fe67e1/jobs/17379

First try to adjust the circleci configuration to run unit tests step so that it can run faster on CircleCI. (this might be a per-step or per-pipeline configuration or something in between)

QA Notes

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DEV Notes

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Design

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See full ticket and images here: FECFILE-2939

[exalate-issue-sync]

Joshua Coombs commented:
[Docker layer caching for CircleCI|https://circleci.com/blog/config-best-practices-docker-layer-caching/]