Checking category IDs in mappings are valid

[Catherine-Fu]

Checking categories IDs used in mappings are present in corresponding category files

• add this consistency validation in an integration testing
• this validation takes 0.29s

Testing result:

[tylerrowsell]

Looks like this test will do the trick. I added some nits/notes on how you might be able to cleanup the code to make it more ruby like and DRY it up a little bit.