#365 Add Delta / Tolerance for TestCase

[Mathias B]

Standard asserts support setting a tolerance for the test result, with a Delta argument, or using Within.
The same feature would be useful for TestCase attributes, because these are commonly used in testing numeric-oriented code, where rounding issues are common. Maybe something along the lines of [TestCase(argument, argument, ... Result = result, Tolerance = 0.01)].