Move tests next to implementation, and fix filename

[benchristel]

Summary:

The tests for axis ticks looked lonely hanging out in __tests__, so I
moved them next to the associated production code. I also renamed one
file from tests.ts to test.ts - the name was preventing Jest from
running the tests in that file.

Issue: none

Test plan:

yarn test

[khan-actions-bot]

Gerald

Required Reviewers

• @Khan/perseus for changes to .changeset/ten-paws-wash.md, packages/perseus/src/widgets/interactive-graphs/axis-tick-labels.test.ts, packages/perseus/src/widgets/interactive-graphs/axis-ticks.test.ts

---

Don't want to be involved in this pull request? Comment #removeme and we won't notify you of further changes.

[github-actions]

npm Snapshot: Published

Good news!! We've packaged up the latest commit from this PR (493dfc5) and published it to npm. You
can install it using the tag PR1393.

Example:

yarn add @khanacademy/perseus@PR1393

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

./dev/tools/bump_perseus_version.sh -t PR1393

[github-actions]

Size Change: 0 B

Total Size: 850 kB

ℹ️ View Unchanged

Filename	Size
packages/kas/dist/es/index.js	38.2 kB
packages/kmath/dist/es/index.js	4.26 kB
packages/math-input/dist/es/index.js	80.8 kB
packages/math-input/dist/es/strings.js	1.73 kB
packages/perseus-core/dist/es/index.js	906 B
packages/perseus-editor/dist/es/index.js	272 kB
packages/perseus-error/dist/es/index.js	866 B
packages/perseus-linter/dist/es/index.js	21.8 kB
packages/perseus/dist/es/index.js	411 kB
packages/perseus/dist/es/strings.js	3.21 kB
packages/pure-markdown/dist/es/index.js	3.67 kB
packages/simple-markdown/dist/es/index.js	12.4 kB

_{compressed-size-action}

[mark-fitzgerald]

Loving the spring/summer cleaning!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.81%. Comparing base (7e6ccf3) to head (493dfc5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1393      +/-   ##
==========================================
+ Coverage   69.85%   70.81%   +0.95%     
==========================================
  Files         500      504       +4     
  Lines      104504   104589      +85     
  Branches     5338    11293    +5955     
==========================================
+ Hits        73006    74065    +1059     
+ Misses      31382    30524     -858     
+ Partials      116        0     -116     

see 127 files with indirect coverage changes

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