Maybe like this? And it is easy to read.

[Fact]
    public async Task MyTest()
    {
        await new TestScheduler().WithAsync(async scheduler =>
        {
            // Arrange
            var sut = new MyTestClass();
            var observer = scheduler.CreateObserver<bool>();
            sut.EnableLaser.CanExecute.Subscribe(observer);

            // Act
            sut.EnableLaser.Execute().Subscribe();

            // Assert
            observer.Messages.Should().HaveCount(2);
            observer.Messages.First().Value.Value.Should().BeTrue();
            observer.Messages.Last().Value.Value.Should().BeFalse();
        });
    }