Fix conformance test -- set referrers artifactType with actual type

[MinerYang]

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Comprehensive Summary of your change

Issue being fixed

Fixes #19259

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• Well Written Title and Summary of the PR
• Label the PR as needed. "release-note/ignore-for-release, release-note/new-feature, release-note/update, release-note/enhancement, release-note/community, release-note/breaking-change, release-note/docs, release-note/infra, release-note/deprecation"
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• Made sure tests are passing and test coverage is added if needed.
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[codecov]

Codecov Report

Merging #19260 (22b249c) into main (c7e2529) will increase coverage by 3.18%.
Report is 13 commits behind head on main.
The diff coverage is 45.45%.

@@            Coverage Diff             @@
##             main   #19260      +/-   ##
==========================================
+ Coverage   67.39%   70.57%   +3.18%     
==========================================
  Files         993      749     -244     
  Lines      108888    95488   -13400     
  Branches     2751        0    -2751     
==========================================
- Hits        73382    67393    -5989     
+ Misses      31561    24456    -7105     
+ Partials     3945     3639     -306     

Flag	Coverage Δ
unittests	70.57% <45.45%> (+3.18%)	⬆️

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Files Changed	Coverage Δ
src/controller/artifact/processor/default.go	70.83% <37.50%> (-0.60%)	⬇️
src/server/registry/referrers.go	68.14% <50.00%> (ø)
src/server/middleware/subject/subject.go	49.31% <100.00%> (ø)

... and 254 files with indirect coverage changes

[MinerYang]

closed by opencontainers/distribution-spec@85b910a