Test: Prompt on leaving page while file(s) uploading

[jmcarp]

Users should be asked if they're sure they want to leave the page when changing the URL on the files page if files are currently uploading.

[chennan47]

The test code is already there in the #L1213 in file_tests.py. However, there is a problem with selenium. It always waits for the upload ended then click on the other button which doesn't use to be the case and even tho the upload takes more than 10 secs. Will come back to fix this problem some time later.