The docs say that valid periods when using string value and
period pairs as relative dates are "y", "m", "w", and "d"
https://api.jqueryui.com/datepicker/

Closes gh-2143

Self-closing tags are reported by newer versions of the htmllint
Grunt plugin. They also don't make sense in our HTML files
since they are not XHTML-compliant and they run in HTML mode
anyway.

Some tests were not properly destroying tooltips which made tests
start to fail with the new QUnit.

`QUnit.jsDump` was renamed to `QUnit.dump` in QUnit 2.0.

Also, workaround issues with QUnit Chrome bridge: the Chrome bridge
from `grunt-contrib-qunit` is now getting injected into every single
iframe, including an empty one that has no intention of running QUnit
tests. Since that bridge requires QUnit, it fails with an error
in such cases. Workaround the issue by wrapping the bridge in
another function that bails early if QUnit is not defined.

The `set-output` workflow is deprecated in favor of using environment
variables, see:
https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

Also, fix htmllint lang exclusion patterns.

Changes:
* add `tests/lib/vendor/**/*` to `.eslintignore`
* move `qunit-composite` to `tests/lib` so that we can modify it
* move `qunit-assert-classes` to `tests/lib` so that we can modify it
* move `qunit-assert-close` to `tests/lib` so that we can modify it
* replace `assert.push` with `assert.pushResult`
* remove usage of `QUnit.extend`