"set readOnly" button in test_SimpleTextArea.html broken

[bill]

The "set readonly" button in test_SimpleTextArea.html doesn't do anything, mainly because of a typo in the test file where it specifies false instead of true.

But also:

1. we are missing automated tests for readOnly
2. _FormWidget.js unnecessarily adds readOnly to attributeMap, even though there is a custom setter
3. the custom setter calls dojo.attr(this.focusNode, 'readOnly', value), with a capital O, rather than dojo.attr(this.focusNode, 'readonly', value), is that right?

[Douglas Hays]

(In [24127]) Fixes #12341. Fix test button to set readOnly instead of remove it. Verified readOnly is correct and not readonly. attributeMap already fixed. TextBox? tests already test readOnly extensively and SimpleTextArea? inherits from there.