Fix friend assembly signing for MSTest projects

[David Federman (dfederm)]

Summary

This fixes a signing-key mismatch when building test projects with SignType=Real. MSTest.Sdk marks test host applications with IsTestingPlatformApplication=true without setting IsTestProject, so the previous signing condition incorrectly assigned MicrosoftCorporation.snk to test assemblies. The updated condition excludes testing platform applications from the production-key path, allowing them to use the test signing key instead.

Production projects still resolve to MicrosoftCorporation.snk; only IsTestingPlatformApplication=true builds are diverted to Test.snk, matching the existing InternalsVisibleTo public key.

Validation

Test / build	Result
dotnet build MSBuildCache.sln --configuration Release --no-restore -p:SignType=Real -p:MicroBuild_SigningTargetsImported=true	Passed
Property evaluation for signing key selection	MSTest projects use Test.snk; production projects use MicrosoftCorporation.snk
dotnet test for net9.0	Passed