> C# has the internal scope for each assembly, but this breaks in combination with unit tests placed in seperate testing assemblies.
This is a limitation imposed by the IDE, not the language. There's nothing stopping you from compiling code and tests into the same dll that you unit test. Likewise there is no need to separate code from the tests (apart from different files), they can simply not be included in release builds.
Source layout structure does not have to be a 1 to 1 mapping of the output structure.
Not keeping tests separate from the tested code can lead to chaos in the long term. It provides an incentive to blur the lines in inappropriate ways, e.g. by adding helper code for tests to the code under test etc.
This is a limitation imposed by the IDE, not the language. There's nothing stopping you from compiling code and tests into the same dll that you unit test. Likewise there is no need to separate code from the tests (apart from different files), they can simply not be included in release builds.
Source layout structure does not have to be a 1 to 1 mapping of the output structure.