[tests-only] Add tests for upload and create folder in a public link

These new and changed scenarios use core test steps added in PR https://github.com/owncloud/core/pull/38204

1) adjust the existing scenario for downloading a file in a public link folder. It was previously strange - it tried to download using a step for the "last publicly shared file" and was probably attempting to fetch the actual public link folder. Now it clearly downloads a file that is in the public link foldder.

2) Add scenarios to upload a file into a public link folder, and create a folder in a public link folder. They check that brute force protection locks those things out after 3 attempts with the wrong password.

owncloud / brute_force_protection

[tests-only] Add tests for upload and create folder in a public link #147