🧪 TESTS: Add tests for JS tab functions

[foster999]

Got a few tests working here, but additional steps needed before it's complete:

• [x] Add test for #105
• [x] Get testing to run with tabs.js in it's current location. Tests currently require you to move tabs.js to the project root...
• [x] Add JS testing to CI workflow

[codecov[bot]]

Codecov Report

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

Project coverage is 97.16%. Comparing base (0f3cbbe) to head (32fca99). Report is 49 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #109 +/- ## ======================================= Coverage 97.16% 97.16% ======================================= Files 2 2 Lines 212 212 ======================================= Hits 206 206 Misses 6 6 ``` | [Flag](https://app.codecov.io/gh/executablebooks/sphinx-tabs/pull/109/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks) | Coverage Δ | | |---|---|---| | [pytests](https://app.codecov.io/gh/executablebooks/sphinx-tabs/pull/109/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks) | `97.16% <ø> (ø)` | | 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=executablebooks#carryforward-flags-in-the-pull-request-comment) to find out more.

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[foster999]

Actions seems to run properly, so this should be good to include now. It doesn't alter any functionality, so shouldn't need a new version from just this