[Dreaming] Clarify: don't hardcode backslash path separators in test XML

[github-actions]

What changed

Added one clarifying sentence to .github/instructions/tests.instructions.md, appended to the existing line about NormalizeSlashes/cross-platform path comparisons: a note that item/metadata path values in inline test XML must not hardcode \ as a separator (invalid on Unix) and should be built with Path.Combine instead.

Why

This is a recurring cross-platform correctness miss in test PRs this week:

• PR Resolve PathLike MatchOnMetadata against the thread working directory #14916 ("Resolve PathLike MatchOnMetadata against the thread working directory"): a bot review flagged hardcoded \ in inline test XML metadata (M1='...\sub\file.txt'), which produces invalid paths on Linux/macOS and breaks the intended assertion.
• PR Fix NormalizePath and other intrinsic functions in multithreaded mode #14875 ("Fix NormalizePath and other intrinsic functions in multithreaded mode"): review discussion (Copilot + JanProvaznik) centered on the same class of path-separator/normalization confusion recurring "in the same file."

Both PRs are in the multithreading/path-normalization area and both needed a reviewer to catch a hardcoded-backslash-in-test-XML issue — exactly the kind of miss a short instruction-file note prevents up front.

I checked the existing tests.instructions.md content and confirmed no existing bullet already states this — the nearest related guidance (NormalizeSlashes for cross-platform path comparisons) covers comparison but not construction of test XML path values, so this is a minimal in-place amendment rather than a duplicate.

Reviewers (core MSBuild team)

Suggested reviewers — could not verify individual @dotnet/kitten team membership via the API (404), falling back to team mention: @dotnet/kitten

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