Add xunit trait attributes to 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 added xunit traits attribute to the tests. I think this makes it easier to filter for specific tests. For example to run only tests for a specific format:

dotnet test -c Debug -f netcoreapp3.1 --filter "Format=Png"

Also you can filter by traits in the test explorer:

[codecov]

Codecov Report

Merging #1455 (455088e) into master (2ffc4e8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1455   +/-   ##
=======================================
  Coverage   83.66%   83.66%           
=======================================
  Files         736      736           
  Lines       32012    32012           
  Branches     3609     3609           
=======================================
  Hits        26782    26782           
  Misses       4516     4516           
  Partials      714      714           

Flag	Coverage Δ
unittests	83.66% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 14c541c...455088e. Read the comment docs.

[antonfirsov]

LGTM.