Improve JoinedListIterator code coverage

[Ivruix]

Added tests for the JoinedListIterator class.

One of the tests I wanted to add was the following:

@Test  
void previousIndexTest() {  
	final ListIterator<Integer> joined = new JoinedListIterator<>(  
		new ListOf<>(1).listIterator(),  
		new ListOf<>(2).listIterator(),  
		new ListOf<>(3).listIterator()  
	);  
	joined.next();  
	new Assertion<>(  
		"Must return index of the previous element",  
		joined.previousIndex(),  
		new IsEqual<>(0)  
	).affirm();  
	joined.next();
	new Assertion<>(  
		"Must return index of the previous element",  
		joined.previousIndex(),  
		new IsEqual<>(1)  
	).affirm();  
}

Surprisingly, the test failed. Currently, the previousIndex() method returns -1 for the first call and 0 for the second call, which contradicts the expected behavior. My suspision is that the implementation of previousIndex is incorrect.

@Override  
public int previousIndex() {  
	int previousidx = -1;  
	if (this.hasPrevious()) {  
		previousidx = this.cursor.get() - 1;
	}  
	return previousidx;  
}

The line previousidx = this.cursor.get() - 1 should likely be previousidx = this.cursor.get().

If my suspicion is confirmed, I will fix the implementation of previousIndex() and add the mentioned test to ensure the corrected behavior is validated.

[Ivruix]

@yegor256 please take a look

[Ivruix]

@yegor256 please take a look

[yegor256]

@Ivruix thanks!

[Ivruix]

@yegor256 can you also please check whether I'm correct about previousIndex function? Off by one error like this might cause a lot of trouble for someone.