CSS: Make the reliableTrDimensions support test work with Bootstrap C…

…SS (3.x version)

Bootstrap 5 includes the following CSS on the page:

```css
*,
*::before,
*::after {
  box-sizing: border-box;
}
```

That threw our `reliableTrDimensions` support test off. This change fixes the
support test and adds a unit test ensuring support test values on a page
including Bootstrap 5 CSS are the same as on a page without it.

Fixes gh-5270
Closes gh-5279
Ref gh-5278

Gruntfile.js

@@ -89,6 +89,10 @@ module.exports = function( grunt ) {
 					destPrefix: "external"
 				},
 				files: {
+					"bootstrap/bootstrap.css": "bootstrap/dist/css/bootstrap.css",
+					"bootstrap/bootstrap.min.css": "bootstrap/dist/css/bootstrap.min.css",
+					"bootstrap/bootstrap.min.css.map": "bootstrap/dist/css/bootstrap.min.css.map",
+
 					"core-js/core-js.js": "core-js/client/core.min.js",
 					"core-js/LICENSE.txt": "core-js/LICENSE",