Increase testify/assert test coverage from 86.6% to 97.1%, fix bugs.

Extend tests for `NotEqual`, `Len` and add tests for `Condition`, `InEpsilon`.
Add tests for `Assertion.Condition`, `Assertion.InEpsilon`, `Assertion.InDelta`.
Fix a bug for `Assertion.InDelta`, `Assertion.InEpsilon` having incorrect signatures.