Translation:The test yesterday was not easy at all.
やさしくないです。 This would be the negated present tense.
やさしくなかったです。 This would be the negated past tense. Not impolite because of the です.
やさしくありません。 This is how you would find it in books, but I doubt any Japanese person talks like that among themselves. It would make you sound awkward in daily conversations.
The conjugation is wrong because ない itself follow the conjugation rule of i-adjectives. You can instead use another form "やさしくなかったです". Actually this form was grammatically incorrect decades ago but is now much more common than -くありませんでした. The latter is used more in formal conversations.