[BEAM-13866] Add miscellaneous exec unit tests

[jrmccluskey]

Adds small unit tests. Adds checks for the discard node (all no-ops,) adds a session windowing case to the AssignWindows testing, and adds simple windowing and window mapping unmarshalling tests.

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Codecov Report

Merging #17363 (55ce567) into master (1d3d43b) will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #17363      +/-   ##
==========================================
+ Coverage   74.00%   74.06%   +0.06%     
==========================================
  Files         685      685              
  Lines       89720    89720              
==========================================
+ Hits        66393    66448      +55     
+ Misses      22167    22100      -67     
- Partials     1160     1172      +12     

Flag	Coverage Δ
go	49.99% <ø> (+0.21%)	⬆️

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

Impacted Files	Coverage Δ
sdks/go/pkg/beam/core/runtime/xlangx/registry.go	47.32% <0.00%> (ø)
sdks/go/pkg/beam/core/runtime/exec/translate.go	14.50% <0.00%> (+7.53%)	⬆️
sdks/go/pkg/beam/core/runtime/exec/window.go	49.01% <0.00%> (+9.80%)	⬆️
sdks/go/pkg/beam/core/runtime/exec/discard.go	71.42% <0.00%> (+71.42%)	⬆️

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[riteshghorse]

LGTM

[riteshghorse]

R: @lostluck

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[lostluck]

LGTM