Add additional pixel conversion tests

[brianpopow]

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

I have noticed that some pixel format are missing some test coverage. This PR adds additional unit tests to increase test coverage for the pixel format tests.

[codecov]

Codecov Report

Merging #903 into master will increase coverage by 0.26%.
The diff coverage is 99.79%.

@@            Coverage Diff             @@
##           master     #903      +/-   ##
==========================================
+ Coverage   89.28%   89.55%   +0.26%     
==========================================
  Files        1031     1031              
  Lines       45631    46115     +484     
  Branches     3261     3261              
==========================================
+ Hits        40741    41297     +556     
+ Misses       4180     4108      -72     
  Partials      710      710

Impacted Files	Coverage Δ
...geSharp/PixelFormats/PixelImplementations/Byte4.cs	84.61% <0%> (+25.64%)	⬆️
tests/ImageSharp.Tests/PixelFormats/Byte4Tests.cs	100% <100%> (ø)	⬆️
tests/ImageSharp.Tests/PixelFormats/Gray8Tests.cs	100% <100%> (ø)	⬆️
...eSharp.Tests/PixelFormats/NormalizedShort4Tests.cs	100% <100%> (ø)	⬆️
...geSharp.Tests/PixelFormats/NormalizedByte4Tests.cs	100% <100%> (ø)	⬆️
tests/ImageSharp.Tests/PixelFormats/Argb32Tests.cs	100% <100%> (ø)	⬆️
...sts/ImageSharp.Tests/PixelFormats/Bgra4444Tests.cs	100% <100%> (ø)	⬆️
tests/ImageSharp.Tests/PixelFormats/Bgr565Tests.cs	100% <100%> (ø)	⬆️
...sts/ImageSharp.Tests/PixelFormats/Bgra5551Tests.cs	100% <100%> (ø)	⬆️
.../ImageSharp.Tests/PixelFormats/Rgba1010102Tests.cs	100% <100%> (ø)	⬆️
... and 14 more

---

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[codecov]

Codecov Report

Merging #903 into master will increase coverage by 0.14%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #903      +/-   ##
==========================================
+ Coverage   89.33%   89.47%   +0.14%     
==========================================
  Files        1029     1029              
  Lines       45708    45975     +267     
  Branches     3260     3260              
==========================================
+ Hits        40833    41137     +304     
+ Misses       4174     4137      -37     
  Partials      701      701

Impacted Files	Coverage Δ
tests/ImageSharp.Tests/PixelFormats/Bgr565Tests.cs	100% <100%> (ø)	⬆️
...sts/ImageSharp.Tests/PixelFormats/Bgra4444Tests.cs	100% <100%> (ø)	⬆️
tests/ImageSharp.Tests/PixelFormats/Byte4Tests.cs	100% <100%> (ø)	⬆️
...sts/ImageSharp.Tests/PixelFormats/Bgra5551Tests.cs	100% <100%> (ø)	⬆️
.../ImageSharp.Tests/PixelFormats/Rgba1010102Tests.cs	100% <100%> (ø)	⬆️
...harp/PixelFormats/PixelImplementations/Bgra5551.cs	84.21% <0%> (+10.52%)	⬆️
...p/PixelFormats/PixelImplementations/Rgba1010102.cs	82.05% <0%> (+17.94%)	⬆️
...geSharp/PixelFormats/PixelImplementations/Byte4.cs	79.48% <0%> (+20.51%)	⬆️
...eSharp/PixelFormats/PixelImplementations/Bgr565.cs	84.21% <0%> (+21.05%)	⬆️
...harp/PixelFormats/PixelImplementations/Bgra4444.cs	83.78% <0%> (+27.02%)	⬆️

---

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[JimBobSquarePants]

This is fantastic. Thanks for the additional tests and the bug fix. I'll get it merged once our CI catches up. 👍