[#24789] Add message for new prism users, in case of failures that might have worked on direct runner.

[lostluck]

If a BeamGo test user has been relying on the direct runner, they might encounter failures when upgrading to 2.50. This message is for when prism is used as the default runner, and that the runner may not have been intentionally prism.

This avoids hiding the error received, and allows time pressed users to have a quick bandaid for their test failures.

We'll keep this message around for several releases, at which point it will be removed.

Fixes #24789.

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[codecov]

Codecov Report

Merging #27886 (4ba4755) into master (3b112e8) will decrease coverage by 0.01%.
Report is 2 commits behind head on master.
The diff coverage is 54.54%.

@@            Coverage Diff             @@
##           master   #27886      +/-   ##
==========================================
- Coverage   70.86%   70.86%   -0.01%     
==========================================
  Files         861      861              
  Lines      105010   105020      +10     
==========================================
+ Hits        74413    74418       +5     
- Misses      29038    29041       +3     
- Partials     1559     1561       +2     

Flag	Coverage Δ
go	53.70% <54.54%> (-0.01%)	⬇️

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

Files Changed	Coverage Δ
sdks/go/pkg/beam/testing/ptest/ptest.go	54.79% <54.54%> (-0.77%)	⬇️

... and 1 file with indirect coverage changes

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[jrmccluskey]

LGTM