Supply a runtests.sh in the tests directory.

[Keryn Knight]

Every time I think about contributing, literally the first thing I want to do is do a git clone and run the tests.
This is currently annoying, because it requires (according to the documentation), my either adding the newly cloned directory to my path permanently (and knowing how to do that. I do, but it raises the barrier to entry for others) or having cloned it via git which intrinsically puts it on the path and smashes it into a src directory.

Instead, I have to go and look in the documentation (which is squirrelled away via Documentation -> How to get involved -> Write unit tests) and find PYTHONPATH=..:$PYTHONPATH ./runtests.py because there's literally no chance I'm going to remember the specifics of that every time I sporadically want to get involved (questions in could go through my mind: do I need to put a semi-colon in? Do I need to export it? do I need to provide a specific settings file?)

Executing ./runtests.py without the incantation helpfully just errors with:

Traceback (most recent call last):
  File "./runtests.py", line 13, in <module>
    import django
ImportError: No module named django

or if you're in a virtualenv with an older extant copy of Django (by way of example) ...

Traceback (most recent call last):
  File "./runtests.py", line 18, in <module>
    from django.test.runner import default_test_processes
ImportError: cannot import name 'default_test_processes'

rather than let me know how to get back on the straight and narrow.

The barrier to entry is too high. If runtests.py cannot execute, a better error message would be useful. Ostensibly something like the following would probably work, for the fresh clone scenario (but not the already-installed-on-path one):

try:
    import django
except ImportError:
    sys.stdout.write("Some error message explaining how to run the thing\n")
    sys.exit(1)

Really though, a single entry point ./runtests.sh which passes through arguments ($@ I think?) that might want to be changed would make life easier from both a user and documentation standpoint ("just run the shell script in the tests directory")

(Additional note: there is a README.rst file in the tests directory, but it a) says to cd to a directory which does not exist if you're already in the directory [which you probably are to run the tests the documented way] and b) requires installing requirements files that the quickstart doesn't)