Disable some slow low value lints for tests in tree

[CraigMacomber]

Description

This improves lint time for tree from 53 seconds to 44 (or 27 to 22.5 when combined with #27431

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Based on the diff (14 lines, 1 files), I've queued these reviewers:

• Correctness — logic errors, race conditions, lifecycle issues
• Security — vulnerabilities, secret exposure, injection
• API Compatibility — breaking changes, release tags, type design
• Performance — algorithmic regressions, memory leaks
• Testing — coverage gaps, hollow tests

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Pull request overview

This PR reduces lint time for the @fluidframework/tree package by disabling several expensive ESLint rules specifically for test files (src/test/**/*), where their signal is considered low relative to their runtime cost.

Changes:

• Disable promise-related lint rules in test files (@typescript-eslint/no-misused-promises, @typescript-eslint/no-floating-promises).
• Disable additional high-cost rules for tests (@typescript-eslint/strict-boolean-expressions, import-x/namespace, unicorn/better-regex).
• Add inline documentation grouping these disabled rules as a performance-focused override for tests.