Use reflection API instead of array diffs.

Use the built-in reflection API's to choose methods. This has the nice
advantage of letting the test generator include methods that are
overriding methods in the parent class. This covers handy things like
Table::initialize() which also fixes generated tests creating warnings.

Refs #4892
Improve test template so it always includes one method.

Having at least one test method in a generated file will prevent errors
from PHPUnit about empty test classes.

Refs #4892
Merge pull request #4915 from cakephp/issue-4892

Update test generation to always include at least one method