The "seq" type is just an alias for "IEnumerable", and all the F# collection types implement the IEnumerable interface, so you can treat e.g. a list of integers as a seq of integers.
Yes, you can use things like NUnit (we actually use this on the F# team for some of our unit testing). There are also some unit testing tools developer in F#, like FsCheck. You might check out http://stackoverflow.com/questions/1989487/f-development-and-unit-testing to learn more.