Add coverage check to tests (#1669)

* check coverage

* Just use `Number.isNaN` as it's widely available

* add test for expected value error in `closeTo`

* raise coverage number to 99

lib/chai/utils/isNaN.js

@@ -4,23 +4,4 @@
  * MIT Licensed
  */
 
-/**
- * ### .isNaN(value)
- *
- * Checks if the given value is NaN or not.
- *
- *     utils.isNaN(NaN); // true
- *
- * @param {unknown} value The value which has to be checked if it is NaN
- * @returns {boolean}
- * @name isNaN
- * @private
- */
-function _isNaN(value) {
-  // Refer http://www.ecma-international.org/ecma-262/6.0/#sec-isnan-number
-  // section's NOTE.
-  return value !== value;
-}
-
-// If ECMAScript 6's Number.isNaN is present, prefer that.
-export const isNaN = Number.isNaN || _isNaN;
+export const isNaN = Number.isNaN;