Tests: Add theme keyboard navigation tests

[sirreal]

This file was added in #9014 but missing from r60516.

While reviewing another PR, I noticed Failed to load resource: net::ERR_FILE_NOT_FOUND errors in the qunit tests and investigated. It led me to this line which references a file that doesn't exist in the repo:

wordpress-develop/tests/qunit/index.html

Line 150 in f0aa4ec

<script src="wp-admin/js/theme.js"></script>

See https://core.trac.wordpress.org/ticket/63126.

Trac ticket: https://core.trac.wordpress.org/ticket/64225

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[github-actions]

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Props jonsurrell, mukesh27.

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[sirreal]

Testing can confirm that there are no Failed to load resource: net::ERR_FILE_NOT_FOUND messages on CI:

https://github.com/WordPress/wordpress-develop/actions/runs/21587508566/job/62199177243?pr=10847

Trunk does have them:

Running "qunit:files" (qunit) task
Testing tests/qunit/compiled.html JQMIGRATE: Migrate is installed, version 3.4.1
Failed to load resource: net::ERR_FILE_NOT_FOUND
.....................................................................................................................................................................................................................OK
Testing tests/qunit/index.html JQMIGRATE: Migrate is installed with logging active, version 3.4.1
Failed to load resource: net::ERR_FILE_NOT_FOUND
.....................................................................................................................................................................................................................OK

[github-actions]

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[sirreal]

I'm surprised this was not caught by quint and reported as an error:

https://qunitjs.com/api/extension/QUnit.onUncaughtException/

[mukeshpanchal27]

I'm surprised this was not caught by quint and reported as an error:

https://qunitjs.com/api/extension/QUnit.onUncaughtException/

+1 Let's explore why it was not shown the error and commit this one