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

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

Closes #13583
Closes #13663