Assertion exception type configuration

[YevgeniyShunevych]

Add a possibility to configure type of assertion exception.

Add a couple of methods to AtataContextBuilder:

AtataContext.Build().
    UseAssertionExceptionType(typeof(NUnit.Framework.AssertionException)).
    // Or
    UseAssertionExceptionType<NUnit.Framework.AssertionException>().

It can be helpful, for example, to override the exception type to NUnit.Framework.AssertionException to get working NUnit's test retry functionality (see NUnit.Framework.RetryAttribute).