[release/10.0] Enable class-level parallelism in Process tests

[steveisok]

Fixes Issue
N/A: test-only CI throughput and ambient-state isolation; no release timeout incidence has been established.

main PR
#135160, merged squash commit 780c55a27764cb5729babfcfef8de849cdf3fcaf.

Description

Backport the final merged approach to release/10.0, based on 4c61b1441b4b11274d6f8b4b3bebc1147e437e78. Release has the same assembly-wide collection and host environment/console/current-directory mutations. Isolating those mutations allows default per-class collections on Windows/Linux, improving throughput/headroom without increasing timeouts.

Desktop macOS retains the original assembly-wide serialization pending #135294; this does not fix or close that issue. The target-based guard applies to both Mono and CoreCLR. Browser's assembly skip and mobile behavior are unchanged.

Six test files only. Conflict resolutions preserve release tests/APIs and omit main-only context. The Windows first-chance-exception test hunk is omitted because that test does not exist on release. The Unix directory-collision test retains its platform/outer-loop traits and now intentionally requires RemoteExecutor.IsSupported, matching main.

Customer Impact

N/A: test-only CI efficiency/isolation. No shipped library binaries, product behavior, package versions, or pipeline changes. This does not establish that release CI is affected by #135001 or promise main's measured speedup on the smaller release suite.

Regression

No product regression addressed. The serialization pattern predates this backport.

Testing

Actual release-project MSBuild evaluation on Windows confirmed TARGET_OSX only for TargetOS=osx with the Unix framework, for Mono and CoreCLR. Windows, Linux, Android, MacCatalyst, Browser, and osx with a MacCatalyst framework do not enable the guard.

The documented targeted dotnet build /t:Test command was attempted for the four affected Windows tests but failed before Process-test compilation: eng/targetingpacks.targets:106 reports a missing built shared-framework targeting pack; CoreLib also reports a Runtime Task Host SDK-version metadata error. The manifest requests SDK 10.0.111; the readily installed SDK selected under its roll-forward policy is 10.0.400-preview.0.26312.103. These are local prerequisites/tooling blockers, not established regressions from this patch.

Bounded supplemental Roslyn compilation covered every evaluation-time Compile item in the Windows and Unix source sets, using installed .NET 10.0.8 references, restored release RemoteExecutor/xUnit dependencies, and .NET 10.0.12 for execution. The four affected Windows tests passed: 4 passed, 0 failed, 0 skipped, with real child/grandchild invocations. References to source-project products were supplied by installed/cached .NET 10 assemblies, not freshly built release binaries.

Actual xUnit discovery on Windows found 358 Windows cases in eight class collections; the Unix variants had 407 cases in eight collections for Linux and one shared collection with the macOS guard. Unix results are cross-target metadata checks, not native-platform execution or CI payload counts.

These supplemental checks do not cover the complete official generated entrypoint/assembly metadata, resource-generation, or analyzer pipeline. Repository build/test success and native Linux/macOS/Mono/NativeAOT execution remain unverified. No broad prerequisite build, manual CI/Helix jobs, reruns, or monitoring were started; automatic PR CI must provide full verification.

Risk

Low product risk: only tests change. Intentional Windows/Linux concurrency carries test-scheduling risk, mitigated by moving process-wide environment, console-code-page, and current-directory mutations out of the runner. The Windows child allocates a private console and reports native failures; fixture disposal retains child cleanup. macOS preserves its prior serial scheduling.

Area-owner review and the applicable servicing approval remain required; this PR does not assert either approval or green CI.

Package authoring no longer needed in .NET 9

IMPORTANT: Starting with .NET 9, you no longer need to edit a NuGet package's csproj to enable building and bump the version.
Keep in mind that we still need package authoring in .NET 8 and older versions.

N/A: this backport changes tests only.

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