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
Unskipped more tests.
Remove option `errorMessage` for FormHelper::input().

Similar display can be achieved by modifying template.
Removing test for stuff that is tested somewhere else
Fixing datetime generation with form input()
Merge pull request #2896 from ADmad/3.0-formhelper

3.0 formhelper
Remove lies.
Unskip required attribute tests for input().

Add missing required attribute/classname conventions and remove lengthy
tests.
Remove redundant test.

This test is redundant as the create/update logic is part of the context
classes now.
Remove redundant test.

This test serves no purpose now. With input methods reasonably
idempotent we don't need tests like this.
Remove more redundant tests.

* Remove redundant tests around detecting required attribute.
* Remove test that didn't actually do anything special.
Fix previously skipped test.
Fix typo.
Remove option that no longer works.
Merge pull request #2905 from markstory/3.0-formhelper-test-cleanup

3.0 formhelper input() required attribute
Removed unnecessary assignment