fix(select): re-define ngModelCtrl.$render in the select postLink fn

[Narretz]

Previously, the $render function was re-defined in the select directive's
preLink function. When a select element is compiled, every option
element inside it is linked and registered with the selectCtrl, which
calls $render to update the selected option. $render calls selectCtrl.writeValue,
which adds an unknown option in case no option is selected. In cases where
optgroup elements are followed by a line-break, adding the unknown option
confuses the html compiler and makes it call the link function of the following
option with a wrong element, which means this option is not correctly
registered.
Since manipulation of the DOM in the preLink function is wrong API usage,
the problem cannot be fixed in the compiler.

With this commit, the $render function is not re-defined until the select directive's
postLink function, at which point all option elements have been linked
already.

The commit also changes the toEqualSelectWithOptions matcher to
take selected options in groups into account.

Closes #13583

[Narretz]

This tests the behavior explicitly, while the first test only tests the functionality. I like this one though, because it makes sure we don't call writeValue for each linked option during compilation.