[prism] Simpler io changes to add TestMain.

[lostluck]

The simpler batch of tests for ios that mostly just needed a registration or two and test main. Other transforms with more involved changes will be batched separately.

See #27550 and #24789.

---

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Mention the appropriate issue in your description (for example: addresses #123), if applicable. This will automatically add a link to the pull request in the issue. If you would like the issue to automatically close on merging the pull request, comment fixes #<ISSUE NUMBER> instead.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

To check the build health, please visit https://github.com/apache/beam/blob/master/.test-infra/BUILD_STATUS.md

GitHub Actions Tests Status (on master branch)

See CI.md for more information about GitHub Actions CI.

[codecov]

Codecov Report

Merging #27587 (975dbc7) into master (b2e00ef) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #27587      +/-   ##
==========================================
- Coverage   71.14%   71.10%   -0.05%     
==========================================
  Files         861      861              
  Lines      104560   104646      +86     
==========================================
+ Hits        74389    74406      +17     
- Misses      28621    28683      +62     
- Partials     1550     1557       +7     

Flag	Coverage Δ
go	53.57% <ø> (-0.09%)	⬇️

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

see 10 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[github-actions]

Assigning reviewers. If you would like to opt out of this review, comment assign to next reviewer:

R: @riteshghorse for label go.
R: @ahmedabu98 for label io.

Available commands:

• stop reviewer notifications - opt out of the automated review tooling
• remind me after tests pass - tag the comment author after tests pass
• waiting on author - shift the attention set back to the author (any comment or push by the author will return the attention set to the reviewers)

The PR bot will only process comments in the main thread (not review comments).

[riteshghorse]

LGTM, thanks!