Parallelize AspNetCore 3.x E2E tests to cut CI runtime from ~3.4h to ~12min

[WanjohiSammy]

Issues

The Core E2E 3x Tests pipeline step (Microsoft.Test.E2E.AspNetCore3x.OData.csproj) runs the ~4900-test suite serially for ~3.4 hours on the Microsoft-hosted agent.

Description

Enable xUnit parallelism for the AspNetCore 3.x E2E project only and harden test isolation so the suite is stable under concurrency. Test-only change - no product code is modified.

Root cause of the 3.4h: WebHostTestFixture applied [assembly: CollectionBehavior(CollectionPerAssembly, MaxParallelThreads = 1)], forcing fully serial execution. Naive parallelism deadlocks instead, because many EF test classes concurrently DROP/CREATE the single shared (LocalDb)\MSSQLLocalDB instance.

Changes:

• Parallelism, scoped to 3.x (#if NETCORE && !NETCORE2x) with a moderate cap: [assembly: CollectionBehavior(MaxParallelThreads = 4)]. Classic (.NET Framework) and AspNetCore 2.x keep the original serial behavior.
• Serialize shared-state tests into one [Collection("Database")] (24 classes) so they never run concurrently: all EF/LocalDB-backed classes, plus the former Aggregation (shared static state) and TimeZoneTests (process-global TimeZoneInfo) collections - folded together because DateAndTimeOfDayTest is both EF- and TimeZone-sensitive. In-memory classes stay parallel; Singleton/Batch/JsonLightMetadata collections unchanged.
• Stability hardening: pin ModelBoundQuerySettings CombinedTest model MaxTop (process-global default could leak across concurrent servers); raise PortArranger reserve-retry budget (10 → 200); accept a connection reset in the oversized-header DoS test.

Checklist (Uncheck if it is not completed)

• Test cases added
• Build and test with one-click build and test script passed

Additional work necessary

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