Add DotNetTargetFramework option for .NET testing

[rshade]

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Issue details

Add DotNetTargetFramework option to allow tests to override the target framework specified in .csproj files.

Use Cases

• Testing SDK compatibility across multiple .NET versions without maintaining separate test projects
• CI matrix testing across frameworks (net6.0, net7.0, net8.0, etc.)
• Validating that a locally-built SDK works with different framework versions

Proposed API

test := NewPulumiTest(t,
    "path/to/csharp/project",
    opttest.DotNetTargetFramework("net7.0"),
)

Implementation Notes

• Would modify <TargetFramework> element in .csproj during test setup
• Requires corresponding .NET SDK to be installed on the test machine
• NuGet packages must be compatible with the specified framework
• Test projects should typically specify their framework in .csproj

Priority

Low - test projects should typically specify their framework; framework testing is better handled via separate test projects or CI matrix

Affected area/feature

.NET/C# Testing