Add doc tests to Panic documentation

[killercup]

We recently added these docs:

a-range/src/lib.rs

Lines 54 to 58 in ce391ed

	/// Construct a [Range] that counts up to the given item.
	///
	/// # Panics
	///
	/// This function will panic when trying to create a [Range] where the upper bound is less than the lower bound.

This behavior is tested in this unit test:

a-range/src/lib.rs

Lines 393 to 397 in ce391ed

	#[test]
	#[should_panic]
	fn fail_up_to_invalid_range() {
	from(40).up_to(39);
	}

It is also possible for this to become a doc test (as I've just found out), which would make this more visible.

• Turn fail_up_to_invalid_range and fail_down_to_invalid_range tests into doc tests

[killercup]

cc @babhishek21 -- just found out that should_panic is also available on doc tests :)

[babhishek21]

Ok. On it. ✌️

[babhishek21]

Hey @killercup , do you think it makes sense to keep the original tests after putting in the doc tests?