functional tests: remove shell script wrapper

[redshiftzero]

We should investigate if we can remove the shell script wrapper that runs each functional test one at a time. This was introduced due to issues with state being shared between tests, which we should investigate and resolve anyway.

[redshiftzero]

Two possibilities (there may be more) to what's causing these test failures:

1. We're calling QApplication.exit somewhere, causing subsequent tests to fail: QApplication.exit in test makes subsequent tests fail pytest-dev/pytest-qt#37
2. The pytest-qt docs recommend registering widgets on qtbot (via qtbot.addWidget) such that they are properly closed after test completion, I tried this by registering the main window prior to each test run and it didn't resolve for me though.

[zenmonkeykstop]

To be revisited - functional tests should be capable of being run independently, so this script should not be necessary once that's fixed.