Complete coverage for CheckLuhnSuffixReturnString

[MylesFTOP]

Per issue #20, current coverage for LuhnLibrary is 88.9%, with only one method uncovered - CheckLuhnSuffixReturnString, which is essentially a wrapper for CheckLuhnSuffixReturnBool to return strings of "valid" or "not valid" instead of a boolean.

While the function is relatively trivial (I'm hesitant to compare literals if it doesn't affect the internal logic of the program - this is just standardising the output for re-use, so it's bordering on UI logic), it may be worth making sure that the function is returning a string to catch any regressions (even though the return type is right there in the method name).

Originally posted by @MylesFTOP in #20 (comment)

[MylesFTOP]

Now showing as 100% coverage with commit 083faf8. Closing issue.