Improve test for tools.ensure_dirs

- Add more checks, some of them are deliberately redundant
- Descriptive error messages
- Add comment describing order to “tools.ensure_dirs”
- Avoid copying a list in an assertion in “tools.ensure_dirs”

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>