Remove unused fixtures.

Kill off a pile of fixtures we are no longer using.

Refs #2345
Remove AclShellTest case.

The AclShell has been deleted for a while, remove the test case (which
was skipped) as well.
Merge pull request #2642 from cakephp/3.0-remove-fixtures

3.0 remove fixtures
Make error message for missing fixture easier to understand.

Telling both the class, fixture reference and test case will help
people solve problems a bit quicker.
Merge pull request #2640 from markstory/3.0-radio-widget

3.0 - Create radio widget
Start implementing checkbox widget.

Basic features and attributes are working, next up is making
checked property smarter.
Expand logic to detect checked.

Since value is a primitive attribute, re-using it for checked/selected
state is a bit of a bad idea. This will probably lead to refactoring in
selectbox and radio as well.
Convert value to selected in SelectBox.

Having consistency between checkbox, select, and radio is more important
than keeping the code stable at this point.
Convert Radio to use selected instead of value.

Having consistency between components is important at this stage,
and will be important later as well. Value is harder to use as it over
laps with checkbox values.
Unset attribute instead of exclude it.
Update doc blocks and coding standards.