Add DotNetBuildConfiguration option for .NET testing

[rshade]

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Issue details

Add DotNetBuildConfiguration option to allow tests to specify Debug vs Release build configuration for .NET projects.

Use Cases

• Testing performance-sensitive code where Release optimizations matter
• Testing conditional compilation symbols that differ between Debug/Release
• Validating that Release builds work correctly with local SDK references

Proposed API

test := NewPulumiTest(t,
    "path/to/csharp/project",
    opttest.DotNetBuildConfiguration("Release"),
)

Implementation Notes

• Could set MSBuild Configuration property via environment variable
• Alternative workaround: Users can use opttest.Env("Configuration", "Release")

Priority

Low - current workaround exists via opttest.Env(), no clear testing use case identified yet

Affected area/feature

.NET/C# Testing