[test] Use `with open` syntax in cwconfig "prefix" tests

[test] Use stdlib unittest.main() in qunit

[web] Clear "pending_others" session key after all relations got processed The RelationField appends data to "pending_others" key while processing its posted information, but this key may already have been removed by the edit controller. So remove the key from session data only when all "others" relations have been processed using a while loop. A test case would be nice, but I could not build one easily unfortunately. Closes #4354551.