* allowClear no longer shifts selections to a new line

This fixes an issue that we have had with the "x" icon used by the
`allowClear` option where selections that just barely interacted
with the position of the "x" icon would be pushed to a new line
that was separate from the normal second line of selections. This
case was pretty rare, because you only had a ~9px area where the
interaction could occur.

The issue was cuased by the "x" icon being sized for the height of
the text in the selection choices, which should be the same as how
the selection choices themselves are sized. Unfortunately this did
not take into account the fact that the selection choices are given
a 1px border which increases their size by 2px, which is what lead
to the odd behaviour. This behaviour could not be replicated without
the 1px border because the height would then line up correctly.

The issue can be fixed by adding a 2px margin to the bottom of the
"x" icon, which would force overlapping selections on to the correct
second line of selections. This was the method that many users have
been using to correct this issue, but was not the method we chose to
use. A 1px padding has been added to the "x" icon instead, which
should expand the touch area of the "x" by a little while also
increasing the height of the "x" by enough to prevent the overlapping.

Fixes select2#4470

* Remove hard-coded height in tests

Because tests are executed on different browsers, and because each
browser sets their own line height, we cannot depend on the height
of the default Select2 being consistent across browsers. As a result,
we must write our tests to calcualte the expected height based on
known data. In the case of this test, we can calculate ahead of time
what two rows of selections is supposed to look like, instead of the
edge case that we can otherwise encounter.