2015-21.py: Add item_combinations stub and test
2015-21.py: Add item_combinations stub and test

Not convinced this test is right, or the design of
item_combinations is right.

robjwells/adventofcode-solutions,robjwells/adventofcode-solutions,robjwells/adventofcode-solutions,robjwells/adventofcode-solutions,robjwells/adventofcode-solutions,robjwells/adventofcode-solutions