move all tests into a subdirectory

[pwnage101]

This should just be the "reorganize tests" part of #71

Changes:

• load test folders are now organized under loadtests/
• unfortunately had to add a line to every locustfile to make the helpers importable. Link to discussion which calls to question the way locust loads the locustfile. TL;DR, when you specify "loadtests/testname" on the locust CLI, locust adds the loadtests/ directory to sys.path then imports the test module as a top-level module; the proposed solution involves getting locustio/locust to merge a simple PR.
• README.rst is updated to reflect the change in structure.

Testing:

I tested a few of the load tests and everything imports fine.

[pwnage101]

FYI @rlucioni @tobz

[tobz]

👍

Not sure how to do the native "approved" action. :)

[tobz]

There we go!

[pwnage101]

thanks, will merge. FYI: locustio/locust#484