[prism] Registrations for filter and stats transform tests.

[lostluck]

Add TestMain overrides for the filter and stats packages.
Register test transforms.

Since filter is a small package, it's been moved to use the generic registration package now. Stats would be a more involved change, but could benefit to a proper move to generics later on.

See #27550 and #24789.

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[codecov]

Codecov Report

Merging #27586 (f9865ce) into master (b2e00ef) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #27586      +/-   ##
==========================================
- Coverage   71.14%   71.14%   -0.01%     
==========================================
  Files         861      860       -1     
  Lines      104560   104571      +11     
==========================================
+ Hits        74389    74392       +3     
- Misses      28621    28622       +1     
- Partials     1550     1557       +7     

Flag	Coverage Δ
go	53.64% <100.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
sdks/go/pkg/beam/transforms/filter/filter.go	100.00% <100.00%> (ø)
sdks/go/pkg/beam/transforms/stats/quantiles.go	82.54% <100.00%> (+0.16%)	⬆️

... and 11 files with indirect coverage changes

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[jrmccluskey]

LGTM