Skip invalid subgroupBroadcast tests

[alan-baker]

Fixes #4532

• Skip tests in compute,active_all and compute,split if the broadcasted ID is not guaranteed to appear in all subgroups
  • For subgroup size 4 this skips 4 subtests (ID=3 for [3,3,3], [15,3,3], [3,15,3], and [3,3,15] sizes)
  • For subgroup size 8 this skips 1 subtest (ID=3 for [3,3,3] size)

Issue: #

---

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 c041f4b):

-webgpu:shader,execution,expression,call,builtin,subgroupBroadcast:compute,all_active:* - 27 cases, 108 subcases (~4/case)
-webgpu:shader,execution,expression,call,builtin,subgroupBroadcast:compute,split:* - 4 cases, 432 subcases (~108/case)
+webgpu:shader,execution,expression,call,builtin,subgroupBroadcast:compute,all_active:* - 27 cases, 216 subcases (~8/case)
+webgpu:shader,execution,expression,call,builtin,subgroupBroadcast:compute,split:* - 4 cases, 864 subcases (~216/case)
-TOTAL: 285738 cases, 2343922 subcases
+TOTAL: 285738 cases, 2344462 subcases

[dneto0]

I don't think this quite fixes it. suggestion below.

[dneto0]

LGTM. Thanks!