[#30083][prism] Stabilize additional teststream cases.

[lostluck]

While working on adding Processing Time handling, it came to light that there were some inconsistent cases WRT handling TestStream based pipelines, in particular when there were no impulses included.

This fix uses the existing watermark hold system to restrain execution until the test stream is ready, and reflect holds based on test stream watermark events properly before their execution.

This discovered flaky fail behavior due to how timers and data outputs were handled in bundle processing, also fixed in this PR.

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[riteshghorse]

LGTM with a few nits, thanks!

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.82759% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 54.39%. Comparing base (a26c505) to head (d17422e).
Report is 37 commits behind head on master.

Files	Patch %	Lines
...g/beam/runners/prism/internal/engine/teststream.go	91.42%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #31046      +/-   ##
==========================================
+ Coverage   54.33%   54.39%   +0.06%     
==========================================
  Files         307      307              
  Lines       38123    38174      +51     
==========================================
+ Hits        20713    20765      +52     
+ Misses      15781    15776       -5     
- Partials     1629     1633       +4     

Flag	Coverage Δ
go	54.39% <94.82%> (+0.06%)	⬆️

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[lostluck]

Nice catches! Thanks. Merging once the recommits pass again.