Add partial Entity save() testing

[MGatner]

Description
Issue #1992 describes the scenario where an entity without every field set will cause non-null database column to error {column_name} cannot be null. This PR will create the necessary tests to catch this scenario prior to providing a fix.

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[MGatner]

Testing with the expected exception "worked" (I got the exception type wrong, but it still threw: https://travis-ci.org/codeigniter4/CodeIgniter4/jobs/529399940). I've removed the expectation and this should now cause a failure because the entity doesn't have every field set.

[MGatner]

Made irrelevant by Entity refactor, #2002