Remove and unskip tests that now pass.

Remove tests that are related to now removed magic in FormHelper. We no
longer have issues with duplicate field names as we don't do nearly as
much magic on them.

Update some tests for secured forms.
Unskip more tests related to secured forms.

These tests all pass with minimal changes. Favour text() over input() as
it is simpler and runs faster.
Remove tests for methods that no longer exist.

domIdSuffix has been moved to Cake\View\Widget\IdGeneratorTrait. These
tests are now defunct.
Remove tests for showParent option.

This option has been removed as it was confusing and the off state was
rarely desirable.
Merge pull request #2895 from markstory/3.0-formhelper-cleanup

3.0 formhelper test cleanup