Better test coverage for SolidBrush + blenders

[antonfirsov]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

• Renamed ImageSharp.Tests/Drawing/BlendedShapes to SolidFillBlendedShapesTests
• Verbose, but descriptive test names
• Removing noise from output image file names
• Assertions through reference images

Having this merged will make merging #552 safer.

[codecov]

Codecov Report

Merging #553 into master will decrease coverage by 0.02%.
The diff coverage is 98.27%.

@@            Coverage Diff             @@
##           master     #553      +/-   ##
==========================================
- Coverage   88.16%   88.14%   -0.03%     
==========================================
  Files         838      838              
  Lines       34974    35036      +62     
  Branches     2529     2532       +3     
==========================================
+ Hits        30836    30883      +47     
+ Misses       3394     3393       -1     
- Partials      744      760      +16

Impacted Files	Coverage Δ
...Sharp.Tests/Drawing/SolidFillBlendedShapesTests.cs	100% <100%> (ø)
...ts/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs	97.67% <100%> (ø)	⬆️
...harp.Tests/TestUtilities/ImagingTestCaseUtility.cs	96.55% <100%> (+6.73%)	⬆️
...geSharp.Tests/TestUtilities/TestImageExtensions.cs	80.76% <77.77%> (+0.1%)	⬆️
src/ImageSharp/Common/Helpers/ImageMaths.cs	77.41% <0%> (-6.46%)	⬇️
...sing/Dithering/ErrorDiffusion/ErrorDiffuserBase.cs	93.1% <0%> (-3.45%)	⬇️
...Formats/Jpeg/GolangPort/Components/Decoder/Bits.cs	69.69% <0%> (-3.04%)	⬇️
...arp.Tests/TestUtilities/TestEnvironment.Formats.cs	87.87% <0%> (-3.04%)	⬇️
.../ImageSharp.Tests/TestUtilities/TestEnvironment.cs	82.5% <0%> (-2.5%)	⬇️
...c/ImageSharp/Formats/Png/Zlib/ZlibDeflateStream.cs	61.4% <0%> (-1.76%)	⬇️
... and 20 more

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[tocsoft]

lgtm 👍