Disabled RLE BMP tests

[antonfirsov]

In #3111, I'm updating Magick.NET to the latest version. According to dlemstra/Magick.NET#1820, ImageMagick stopped supporting certain invalid BMP files, resulting in test failures.

Disabling these tests, until we figure out a solution:

• BmpDecoder_CanDecode_RunLengthEncoded_8Bit(RLE8Inverted)
• BmpDecoder_CanDecode_RunLengthEncoded_8Bit_WithDelta_MagickRefDecoder(RLE8Cut)

[JimBobSquarePants]

@brianpopow has a workaround in #3096 which I'm ok with.

https://github.com/SixLabors/ImageSharp/pull/3096/changes#diff-d2d5f20c5e00af274852a5ef8a4136e522cf27b22283754437e92c24de93772cR99

[antonfirsov]

I see, it adds a bunch of reference images to handle this, I would rather not duplicate them to my PR. When resolving conflicts, BmpDecoderTests.cs should be taken entirely from @brianpopow 's variant.

[antonfirsov]

To make life even simpler, I decided to revert the Magick.NET update in my PR, #3096 will deal with it anyways.