test(bidi): set channel option in showTraceViewer() fixture

[hbenl]

After #39209 the trace-viewer tests are failing in Firefox Bidi test runs because the defaultLaunchOptions contain channel: 'moz-firefox-nightly-library' and therefore playwright.chromium.launch() throws.

[Skn0tt]

Hmm, looks like we can't fully ignore defaultLaunchOptions. No luck with this patch of mine so far!

[github-actions]

Test results for "tests 1"

4 flaky

⚠️ [chromium-library] › library/popup.spec.ts:258 › should not throw when click closes popup `@chromium-ubuntu-22.04-node20`
⚠️ [firefox-page] › page/page-set-input-files.spec.ts:38 › should upload a folder `@firefox-ubuntu-22.04-node20`
⚠️ [playwright-test] › ui-mode-trace.spec.ts:384 › should reveal errors in the sourcetab `@macos-latest-node20`
⚠️ [playwright-test] › ui-mode-trace.spec.ts:700 › should indicate current test status `@macos-latest-node20`

38521 passed, 843 skipped

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Merge workflow run.

[github-actions]

Test results for "MCP"

1 failed
❌ [chromium] › mcp/tracing.spec.ts:21 › check that trace is saved with --save-trace @mcp-windows-latest

4791 passed, 135 skipped

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Merge workflow run.