FileSystem::tempnam() should use realpath() instead of getDirectoryPath()

Hi @kim.pepper I've added a initial commit to this issue but I'm not sure if we are missing test instructions or may we need add new test cases for it ?

As far I understood it is just a sub task from #833734

Thank you!

I don't believe we have test coverage for this method, so now would be a good time to add it.

I think we should postpone this on 🐛 FileSystem::tempnam() doesn't respect subdirectories for stream wrappers Needs work as that is re-implementing the method without getDirectoryPath() and adding test coverage.

Okay let me know when and if we will proceed at time
thanks