Optimize DotnetSDKSimulation_PostProcessing test (163s → 61s)

[nohwnd]

Summary

Optimizes the slowest acceptance test from 163s → 61s (63% faster).

Changes

• Build once, reuse: Instead of creating and building 5 separate MSTest projects in parallel (5× \dotnet new\ + 5× \dotnet build), create and build one project once and reuse the DLL.
• Reduce runs from 5 to 2: The test validates artifact post-processing/merging — 2 runs is the minimum needed to verify merge behavior.

Before/After

Metric	Before	After
Duration	163.4s	61.1s
\dotnet new\ calls	5	1
\dotnet build\ calls	5	1
vstest runs	5	2

[Copilot]

Pull request overview

Optimizes the DotnetSDKSimulation_PostProcessing acceptance test runtime by reducing redundant project creation/build work and lowering the number of vstest executions while preserving merge validation.

Changes:

• Create/build a single MSTest project once and reuse its output DLL instead of generating/building multiple projects.
• Reduce parallel vstest runs from 5 to 2 and update the assertion accordingly.
• Use per-run results directories to avoid output collisions during parallel executions.