Replace FluentAssertions with Shouldly or AwesomeAssertions

https://www.reddit.com/r/programming/comments/1i19l11/fluent_assertion_sneakily_changed_from_apache_20/
FluentAssertions 8.0 comes with a new license.  So, instead of upgrading to it, we should migrate to Shoudly once it has a new version released.
An altenative is https://github.com/AwesomeAssertions/AwesomeAssertions, an Apache 2.0 fork of FluentAssertions.