Assert things in specs

These tests pass, but there are still some assertions that are
incorrect. Fixes to come in following commits.

Follow-up to af8f9a7bffa238db2648dfc28f23b7618f6881f2
Assert things in specs

The original context used a model with a string attribute. Override the
`validating_exclusion` method in this context.

Follow-up to af8f9a7bffa238db2648dfc28f23b7618f6881f2
Assert things in specs

The original contexts used models with a string attribute. Override the
`validating_inclusion` method in these contexts.

Follow-up to af8f9a7bffa238db2648dfc28f23b7618f6881f2
Assert things in specs

The original test defined its own model. Do so again.

Partially reverts af8f9a7bffa238db2648dfc28f23b7618f6881f2
Assert things in specs

The original tests had a precision as well as a scale. Do so again.

Follow-up to af8f9a7bffa238db2648dfc28f23b7618f6881f2
Remove duplicated method.
Remove unneeded test that should not have passed.