Remove registry clear calls.

These registry clear calls were useful when TestTask and ModelTask could
be executed multiple times interactively. However, that is not possible
now so they are a liability that causes `bake all` to act
inconsistently.

Refs #3997
Fix ViewTask not correctly handling plugin models.

When baking plugin views we need to use the correct aliases or things
don't work very well.

Refs #4007
Fix $_FILES parsing

The old tests were just straight up wrong. I built them wrong when
converting the code from 2.x to now. In 2.x all files were preceded by
the `data` name which made files parsing much simpler as we had already
stripped off the first name element.

The new method feels better to me as well. It doesn't use reference
arguments and I think is a tiny bit simpler.

Refs #3999
Fix post data that interleaves with FILES not being handled.

Refs #3997
Fix error where multiple files at would be handled incorrectly.

When there are multiple files at the same level the path needs to be
reset on each iteration. Failing to do so causes adjacent files to be
nested incorrectly.
Merge pull request #4012 from cakephp/issue-3997

Fix a bunch of small issues.