Tox should set usedevelop=false

[benjaoming]

Observed behavior

All too often, we are seeing differences between our test environment and the real installed environment.

Two factors:

• Use src/ source structure to avoid kolibri/ folder being in the immediate path when running for instance test commands. Move kolibri/ to src/ #1953 already addresses this
• Tox should build a real .whl, then install it, and then run the tests. This issue addresses this specific need.

Underlying problem is that when we run tests, our development environment is different because it may have a different path access the host system, but also that kolibri/dist may or may not be populated. And the development environment may also have a different set of requirements installed in its virtualenv.

Solution:

1. Set usedevelop=false in tox.ini
2. Build a .whl output in the first stages of Travis and reuse across all test envs. See also Travis' new build stages & build matrix optimizations #2975
3. Remove the need to test the build since it will always be built (aka. conditional tests)

Expected behavior

That we can reproduce the problem reported in #3308 in a Travis VM.. and then fix it!

[benjaoming]

This can also play tricks when testing locally and switching between versions. Assuming that tox runs isolated environments between switching branches is wrong, because *.pyc files remain when checking out different branches.

Because Tox links the git structure into the tox environment, all the .pyc files from previous runtimes are included, for instance migrations and contents of kolibri/dist. These files are used, even though they are orphans.

Moreover, the local kolibri/dist is overwritten when running tox because of the linking - not the one in the tox environment.