Update the default value of ImmediateClock.

[sanghun0724]

To improve consistency with other clocks and readability of the code, Update the default values of the variables in ImmediateClock.

e.g: TestClock initialization

    public var minimumResolution: Duration = .zero
    public private(set) var now: Instant

    private let lock = NSRecursiveLock()
    private var suspensions:
      [(
        id: UUID,
        deadline: Instant,
        continuation: AsyncThrowingStream<Never, Error>.Continuation
      )] = []

    public init(now: Instant = .init()) {
      self.now = now
    }