Allow to start Python test generation for several files

[tyuldashev]

Description

Currently it's not possible to start test generation for several Python files selected in Project toolwindow or for a folder. In case user wants to generate tests for the whole project she has to select files one by one and invoke generation for each of them separately, which could be time consuming.

Expected behavior

The same as in Java I expected that I can start generation (probably sequential) for whole project, folder or just random files selected in Project view.