test(audience-sample): pin frame rate to 1 on Linux for llvmpipe fill rate

[ImmutableJeffrey]

Summary

• Adds LinuxRenderSuppression [SetUpFixture] gated on UNITY_STANDALONE_LINUX.
• On Linux PlayMode runs only: Application.targetFrameRate = 1 and QualitySettings.vSyncCount = 0 for the duration of the suite, restored on teardown.
• Mac and Windows PlayMode runs unaffected.

Why

Tests assert on UI Toolkit's visual-element tree (layout state), not on rasterised pixels. Painting more frames between assertions doesn't help the assertions; it just spends llvmpipe CPU cycles on fragment fill. Capping at 1 fps lets the player do logic work at full speed while skipping most paint cost.

Experiment success criteria

• Unity 6 Linux Mono cell wall time under 18 min (currently around 27 min).
• Unity 2021.3 Linux cells stay at or below their current 4 min.
• All 4 Linux PR cells go green; no UI Toolkit state assertions regress.

🤖 Generated with Claude Code

[ImmutableJeffrey]

Closing without merging. Render suppression made things worse: Unity 2021.3 cells went from 3-4 min to 8-11 min because Application.targetFrameRate=1 starved frame-paced test coroutines. Correct fix is the inverse - decouple test polling from frame pacing entirely. Pursuing that in a new PR.