JpegEncoderTests are not reliable

[antonfirsov]

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of ImageSharp
• I have verified if the problem exist in both DEBUG and RELEASE mode
• I have searched open and closed issues to ensure it has not already been reported

Description

Our current validation logic in JpegEncoderTests is too tolerant, which caused #1549 to go through the quality gate completely unnoticed. The method for calculating the tolerance percentage is result of trial-and error experiments (dealing with platform differences if memory serves well):

ImageSharp/tests/ImageSharp.Tests/Formats/Jpg/JpegEncoderTests.cs

Lines 107 to 128 in 5ab768c

	/// <summary>
	/// Anton's SUPER-SCIENTIFIC tolerance threshold calculation
	/// </summary>
	private static ImageComparer GetComparer(int quality, JpegSubsample subsample)
	{
	float tolerance = 0.015f; // ~1.5%
	if (quality < 50)
	{
	tolerance *= 10f;
	}
	else if (quality < 75 || subsample == JpegSubsample.Ratio420)
	{
	tolerance *= 5f;
	if (subsample == JpegSubsample.Ratio420)
	{
	tolerance *= 2f;
	}
	}
	return ImageComparer.Tolerant(tolerance);
	}

The method above returns a comparer of 15% tolerance, for subsample=420, quality=100, which doesn't really make sense. The image in #1549 (comment) has ~4% difference compared to the image before encoding.