[BigInt tests] ❌ Unary `+`, `-` and `~`

[LiarPrincess]

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Please read the #242 Using tests from “Violet - Python VM written in Swift” before.

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❌ Failures

  func test_unaryMinus() {
    // -9223372036854775808 = Int.min
    // 'Int.min' negation overflows
    let int = -9223372036854775808
    let expected = BigInt(.positive, magnitude: BigIntPrototype.Word(int.magnitude))

    let big = -BigInt(int)
    XCTAssertEqual(big, expected)

    var negated = BigInt(int)
    negated.negate()
    XCTAssertEqual(negated, expected, "\(negated) == \(expected)")
  }