Add testing for installers infra

Contributes to https://github.com/dotnet/source-build/issues/4758

Our infra for building packages and installers for various platforms is rather complex. Making any changes in infra today, requires manual time-consuming builds of various packages in runtime and sdk, to ensure we don't introduce regressions.

The following is needed:
- Uni-tests for installers infra
- Basic testing that would exercise the main code-paths, i.e. creation of DEB/RPM packages and static analysis of package contents and metadata