Test cases for 07_threads/06_interior_mutability should be explained

[iurii-kyrylenko]

The following test case cannot be understood without explanation of how rust handles the temporary values:

100-exercises-to-learn-rust/exercises/07_threads/06_interior_mutability/src/lib.rs

Lines 28 to 33 in d347d1f

	#[test]
	fn it_works() {
	let counter = Rc::new(RefCell::new(0));
	let _ = DropTracker::new((), Rc::clone(&counter));
	assert_eq!(*counter.borrow(), 1);
	}

The use of the wildcard _ as the binding target in the test case it_works() signals to the compiler that the value is only temporary, leading to its immediate drop at the statement's conclusion. In contrast, assigning to a named variable in the test case multiple() ensures the variable's full lifetime within its defining scope:

100-exercises-to-learn-rust/exercises/07_threads/06_interior_mutability/src/lib.rs

Lines 35 to 45 in d347d1f

	#[test]
	fn multiple() {
	let counter = Rc::new(RefCell::new(0));
	{
	let a = DropTracker::new(5, Rc::clone(&counter));
	let b = DropTracker::new(6, Rc::clone(&counter));
	}
	assert_eq!(*counter.borrow(), 2);
	}

To suppress the compiler warning, unused vars a and b should be prefixed by _.