If a user has multiple folders, it is painful to manually expand them all and select the contents of the folder. Dragging and dropping the folder should be possible, this is an intuitive UI design that needs to be implemented.
The suggested functionality could be confusing. Right now, the CrossBrowserTesting editor has groups that behave quite differently from folders in the Project Explorer panel. Taking this into account, it appears that result of moving a folder from the Project Explorer panel to the CrossBrowserTesting editor is unclear and a user's expectations can be wrong.
There is a simple workaround - select all tests within the desired folder in the Project Explorer panel and move this selection to the CrossBrowserTesting editor. This action is pretty straightforward and should not be confusing.