additional tests for take/drop iterators

[stevengj]

This adds a length method for the Take and Drop iterators. I also added a check that the specified length is nonnegative, and a couple of corresponding tests in the test suite.

Note that #12009 is implicated by the added length method, if the underlying iterator is missing length.

[mschauer]

With the typealias InfiniteIterator Union{Cycle,Repeated,Count} of #13276 we could special-case the length of takes of infinite iterators here as well in the meanwhile.

[ararslan]

Still relevant? We have length methods for these now, but the argument validation still seems worthwhile.

[stevengj]

Updated it with just the argument checks and tests.

[ararslan]

Windows failure is a timeout, macOS failure is #26499.