Fix ShellWriter.RmFile(string) for cmd shell

What does this MR do?

1. extract (and expand) OnEachShell into the shellstest package
2. add tests for ShellWriter.RmFile(string)
3. fix RmFile on cmd

Why was this MR needed?

we use rd for RmFile but it cannot delete files, only directories

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

It may fix #3927 (closed)