Add missing transform unit tests

[Hauke Mewes]

Add missing transform unit tests

[Hauke Mewes]

And create test for 2-input-iterator tuple

[Hauke Mewes]

By now, there is only a unit test for the single-input iterator transform, in-place transform.
There needs to be an out-of-place test for the single-input iterator transform, as well as both in-place and out-of-place tests for the double-input iterator transform.

This seems like a simple task, but it would be desirable to have a common code-base for all of the four cases instead of copying the code three times and making the appropriate changes.

[Simeon Ehrig]

done in #40