Prior to this commit, the modified tests used the strict equality
operator to compare computed values with negative zero. Due to the
semantics of that operator, these tests would spuriously pass if the
value under test was in fact positive zero.
Update the tests to be more precise by instead asserting equality with
the `assert.sameValue` utility method (since that method correctly
distinguishes between negative zero and positive zero).
The global isNaN is not precise at all, and Number.isNaN is an ES6 feature that makes it preferrable to use assert's sameValue for NaN values, as it handles it internally using the comparison.