Split test method up.

Smaller tests are easier to debug when they fail.
Start updating inputs().
Get the legend element working again.
Update tests for inputs().

While still failing, these tests should pass eventually.
Add fieldNames() to Form contexts.

Generating inputs based on only an entity requires knowing the field
names, so we'll need a way to reflect that data as well.
Remove redundant test and fix failing tests.

Remove a redundant test for disabling the fieldset, and fix remaining
fails.
Remove legend/fieldset classnames.

This formatting should be handled with templates now.
Remove deprecated parameters and update tests.

Remove deprecated forms of inputs() and update tests. Shuffle tests
around to match the separation added earlier.
The $fields parameter should only handle customizing fields.

The fields parameter should not duplicate the functionality found in the
blacklist parameter. Instead it should only allow customizing the
generated fields.
Merge pull request #2944 from markstory/3.0-form-inputs

3.0 Update FormHelper::inputs()