Simplify test for failing checks

[dupdob]

Each check must have tests to cover success and failure situation(s).
Failure tests require boilerplate code that is errorprone and disheartening.
I propose to add a private custom check:
FailsWithMessage(params string[] message)
It will check that a FluentException is raised and that it contains the expected message, expressed as a list of lines.

[dupdob]

Added. No explicit refactoring/migration of existing tests planned. Migration should be done on a on opportunity basis.