Added SeqTest.verifyIteratorValidity() method

[tlinkowski]

I propose a new private method in SeqTest:

void verifyIteratorValidity(int expectedCount, Seq<T> source, Function<Seq<T>, Seq<?>> operation)

This method verifies whether our non-terminal Seq operations (e.g. limitWhile, partition, splitAt) behave as expected, i.e.:

• whether every call to Spliterator.tryAdvance that returns true actually calls given action
• whether the number of elements fetched from the source sequence is equal to expectedCount
• whether calling redundantly Iterator.hasNext does not fetch any extra element from the source sequence (such calls are valid and should be "safe")

Using this method I detected two defects: #308 and #309.

[lukaseder]

A late thank you very much for this suggestion. I agree with these tests, very useful.

[lukaseder]

Again, I really like this utility. It will help us validate the laziness of a lot of future features... Thanks again!