Fix #1910 KeyboardInterrupt with load shapes

[JevonCowell]

To be honest this is probably more of a bandaid solution, but for dealing with KeyboardInterrupt this is hopefully sufficient. I've added an integration test that raises a KeyboardInterrupt. The logic being that if properly handled, the test will end properly, @events.test_stop.add_listener will execute, and a report will be generated.

[JevonCowell]

Please let me know if any other tests need to be written. This is my first Pull Request on an Open Source Project 😃.

[cyberw]

Thanks! I dont think the flake8 issue is your fault so you can ignore it. In the test, I think you should send the interrupt signal from the test rather than throwing an exception from inside the locustfile. Can you change that? There are some other tests like that in test_main.py

[cyberw]

Nice! For future contributions, use #issuenumber instead of ISSUE-issuenumber: in commit messages to connect them to a specific issue (or leave it out completely, its enough that the PR name and possibly branch name reference it).

I'll squash your commits when merging so I can edit the messages :)

I'll fix the flake8 issue in master.