Fix padding with modal and fixed navbar

[pvdlg]

Fixes #18373, fixes #21700, fixes #20824, fixes #21840.

Now _setScrollbar:

• Save the current body padding-right in this._originalBodyPadding
• Add a pading-right to the body corresponding to the current padding-right + the scrollbar size
• Save the current padding-right of each fixed element in the element's 'padding-right' data-attribute
• Add a pading-right to each fixed element corresponding to the current padding-right + the scrollbar size

And _resetScrollbar:

• Reset the body padding-right to the saved value
• Reset the padding-right of each each fixed element to the saved value

The PR #17536 added 2 unit tests to verify that the padding-right is properly added to the body. However those 2 tests were ineffective because:

• The function _getScrollbarWidth relies on the css class .modal-scrollbar-measure to determine the scrollbar width and therefore the value of the padding-right to add to the body. But the class was not present in the unit tests.
• QUnit doesn't add a scrollbar even when en element is higher than the viewport, the function _getScrollbarWidth was always returning 0px, therefore during a QUnit test the modal plugin was always considering that there is no sidebar, so no padding-right was added to the body

Note: _getScrollbarWidth, is called by _checkScrollbar and sets the current scrollbar width in this._scrollbarWidth. This value is later used by _setScrollbar to determine the width of the padding to add to the body and fixed elements. So _setScrollbar depends on _getScrollbarWidthand .modal-scrollbar-measure.

In order to solve the first issue, this PR add the Bootstrap css to the Unit test so the class .modal-scrollbar-measure and _getScrollbarWidth can work as intended during the unit tests.

To solve the second one, the tests have been modified to simulate a scrollbar by adding 10px to the html element. Then after the modal is opened we verify that a padding-right of 10px (corresponding to the width of the simulated scrollbar) has been added to the body element.
Note: The simulated scrollbar is added to the html element, and after the modal is opened the test verify the padding-right of the body (html != body).

In addition the tests now also check for the padding-right to be applied on the fixed elements.

[Johann-S]

Closed due to the merge of : #18441