plugin: Be more selective about when ! skips validation.

It makes sense to let the bookkeeper skip validations in situations
where the metadata requires information that might not be available
when entered. It does not make sense to skip validations that *must*
be available and affect the structure of the books, like project and
entity.

This commit ensures every plugin hook has a test for flagged
transactions, even for hooks that currently have the desired
behavior where no code changes were required for the test to
pass.