leap: add more test cases

[Emerentius]

The exercise has just 5 test cases. This can easily let some bugs slip through.
This adds a check for a range of years (1600 to 1699) and a few additional centuries. The range is kept short so the user isn't spammed with a huge list of incorrect years with wrong code. It also avoids having the full solution in the tests where students could stumble upon it.

I got this faulty code from a student which passes all the old tests:

year % 4 == 0 && !year % 100 == 0 || year % 400 == 0

[coriolinus]

Looks great! I like the way you implemented the procedural test; it's correct without giving away a complete implementation.

As with your other PRs, the soonest I can merge this is Monday the 27th, but I'm happy if another maintainer merges this before then.

[Emerentius]

I thought on this a bit more. I found it puzzling that bitwise inversion would actually make it work.
-n = !n + 1 (two's complement) or put another way, !n = -n - 1. It just shifts the centuries by 1 and in that case the first modulo 4 check never passes.

So why did I think it passes all the tests? Because all the centuries tests check for false which this happens to return and cargo test -- --ignored doesn't run all tests, only the ignored ones. The very first test would have detected this.

We can ignore this PR but we should get a way to just run all tests in one command.

[petertseng]

For this reason, I have always been forced to run cargo test && cargo test -- --ignored. I continue to look for any better solutions. Let me know of any.

[Emerentius]

I found this relevant issue: rust-lang/rust#50363
A quick search through the source of libtest makes me believe that adding an --all flag shouldn't be difficult. Compiling rustc will be the biggest hurdle for me, but I'll try to tackle this.