update tests for maxChar

[vvnsze]

Improving test coverage because of incorrect solution that was passing all the tests.

I input 'cacacbb' and the output is 'b' when it should be 'c'.
This was the faulty solution that passed the tests:
function maxChar(str) {
var letter;
var count = 0;
str.split('').forEach((item, index) => {
if(letter === undefined || item === letter) {
letter = item;
count++;
} else {
if(count === 1){
letter = item
}
}
})
return letter;
}