Improve fragment shader versions of subgroup tests

[alan-baker]

Incomplete. Posted for testing purposes.

Issue: #

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Requirements for PR author:

• All missing test coverage is tracked with "TODO" or .unimplemented().
• New helpers are /** documented */ and new helper files are found in helper_index.txt.
• Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
• Test have be tested with compatibility mode validation enabled and behave as expected. (If not passing, explain above.)

Requirements for reviewer sign-off:

• Tests are properly located.
• Test descriptions are accurate and complete.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Tests avoid over-parameterization (see case count report).

When landing this PR, be sure to make any necessary issue status updates.

[github-actions]

Results for build job (at 3ef27d2):

-webgpu:shader,execution,expression,call,builtin,quadBroadcast:fragment,all_active:* - 12 cases, 48 subcases (~4/case)
+webgpu:shader,execution,expression,call,builtin,quadBroadcast:fragment,all_active:* - 11 cases, 44 subcases (~4/case)
-webgpu:shader,execution,expression,call,builtin,quadSwap:fragment,all_active:* - 12 cases, 36 subcases (~3/case)
+webgpu:shader,execution,expression,call,builtin,quadSwap:fragment,all_active:* - 11 cases, 33 subcases (~3/case)
-webgpu:shader,execution,expression,call,builtin,subgroupAdd:fragment:* - 36 cases, 36 subcases (~1/case)
+webgpu:shader,execution,expression,call,builtin,subgroupAdd:fragment:* - 33 cases, 33 subcases (~1/case)
-webgpu:shader,execution,expression,call,builtin,subgroupAll:fragment,all_active:* - 12 cases, 180 subcases (~15/case)
+webgpu:shader,execution,expression,call,builtin,subgroupAll:fragment,all_active:* - 11 cases, 165 subcases (~15/case)
-webgpu:shader,execution,expression,call,builtin,subgroupAny:fragment,all_active:* - 12 cases, 180 subcases (~15/case)
+webgpu:shader,execution,expression,call,builtin,subgroupAny:fragment,all_active:* - 11 cases, 165 subcases (~15/case)
-webgpu:shader,execution,expression,call,builtin,subgroupBallot:fragment:* - 9 cases, 108 subcases (~12/case)
+webgpu:shader,execution,expression,call,builtin,subgroupBallot:fragment:* - 9 cases, 99 subcases (~11/case)
-webgpu:shader,execution,expression,call,builtin,subgroupBitwise:fragment,all_active:* - 12 cases, 540 subcases (~45/case)
+webgpu:shader,execution,expression,call,builtin,subgroupBitwise:fragment,all_active:* - 11 cases, 495 subcases (~45/case)
-webgpu:shader,execution,expression,call,builtin,subgroupBroadcast:fragment:* - 12 cases, 48 subcases (~4/case)
+webgpu:shader,execution,expression,call,builtin,subgroupBroadcast:fragment:* - 11 cases, 44 subcases (~4/case)
-webgpu:shader,execution,expression,call,builtin,subgroupElect:fragment:* - 12 cases, 12 subcases (~1/case)
+webgpu:shader,execution,expression,call,builtin,subgroupElect:fragment:* - 11 cases, 11 subcases (~1/case)
 [snip - full report in action logs]
-TOTAL: 280593 cases, 2321592 subcases
+TOTAL: 280577 cases, 2321435 subcases