Add more POST test to check if upload really worked as expected

[hauspie]

Idea is to write two tests in the 02_post test suite for this purpose:

1. first one answer to post request and store the file's content in RAM
2. second one answer to get request and sends back the file's content.

Those tests will need to be executed in the right order, so should be named 01_file_upload and 02_file_check or similar.

Same must also be done to test multiple file upload (in one multipart post request)