Skip tests in ReferenceDecoderBenchmarks

[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

Oops, I left some benchmark tests enabled accidentally in #640

[codecov]

Codecov Report

Merging #709 into master will decrease coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #709      +/-   ##
==========================================
- Coverage   88.64%   88.52%   -0.12%     
==========================================
  Files         890      890              
  Lines       39166    39166              
  Branches     2701     2701              
==========================================
- Hits        34719    34673      -46     
- Misses       3743     3790      +47     
+ Partials      704      703       -1

Impacted Files	Coverage Δ
.../TestUtilities/Tests/ReferenceDecoderBenchmarks.cs	0% <ø> (-100%)	⬇️
src/ImageSharp/Formats/Png/PngDecoderCore.cs	70.17% <0%> (-1.28%)	⬇️

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