WIP: more regression tests

[benchristel]

Issue: XXX-XXXX

Test plan:

[github-actions]

npm Snapshot: Published

Good news!! We've packaged up the latest commit from this PR (0d4a5cd) and published it to npm. You
can install it using the tag PR1956.

Example:

yarn add @khanacademy/perseus@PR1956

If you are working in Khan Academy's webapp, you can run:

./dev/tools/bump_perseus_version.sh -t PR1956

[github-actions]

Size Change: +158 B (+0.01%)

Total Size: 1.29 MB

Filename	Size	Change
packages/perseus/dist/es/index.js	426 kB	+158 B (+0.04%)

ℹ️ View Unchanged

Filename	Size
packages/kas/dist/es/index.js	39 kB
packages/keypad-context/dist/es/index.js	760 B
packages/kmath/dist/es/index.js	4.27 kB
packages/math-input/dist/es/index.js	77.9 kB
packages/math-input/dist/es/strings.js	1.79 kB
packages/perseus-core/dist/es/index.js	1.48 kB
packages/perseus-editor/dist/es/index.js	697 kB
packages/perseus-linter/dist/es/index.js	22.2 kB
packages/perseus/dist/es/strings.js	3.7 kB
packages/pure-markdown/dist/es/index.js	3.66 kB
packages/simple-markdown/dist/es/index.js	12.5 kB

_{compressed-size-action}