Extend filename tests to also include directory names

[PVince81]

Extend test_filenames.py which currently only uploads files into a single directory.

• add test where the generated name is actually the directory
• test uploading into directory called "0" (as additional name pattern)
• test if files can be overwritten (to cover Client does not upload changed file residing in folder with name "0" client#3351 (comment))

@jnfrmarks in general it's a good idea to test with "0" values, because sometimes programming errors in PHP will cause "0" to be evaluated to false or even removed.