refactor tests to be more readable

Here I take advantage of the new API and the assertions from nose.tools
to refactor the tests and make them more readable. They are still green
and should be otherwise unchanged.
make method called by button return True

The method auto_rfq_from_suppliers returned a list of ids of created
RFQs. This list was used by the tests only. In production a method
called by a button should just return True in case of success.

Now the method returns True, and tests are adapted to figure out the
created rfqs from requisition.purchase_ids instead of using the return
value.
improve assertion: compare dict with dict

This way assert_equal gives a very nice failure singling out the
different keys and values in the dictionary.

This is to prepare a test on product quantities, still getting good
failures if for example only the quantities for one line are different.
improve tests to check quantities as well