test_vendor: Add function to extract bytes from iterators

The extract function extracts a number of bytes from the type that the
iterator is pointing to. The iterator will be incremented to the value
after the last byte after extract is called.

Fixed the bounds of the for loop in the preIncrementTest so that it
does not try to dereference the sentinel after the last value.

Test: Passes unit tests in test/iterator_test.cc
Change-Id: I07944641b829c820bbe40d612d7f065e514516b3