Add the option to load testability library by reading an environment variable

Summary

Add the option to load the testability library based on whether an environment variable is set, in addition to the current "-testability" flag.

Rationale

The Autopilot project currently uses the -testability flag when launching Qt apps to load the autopilot testability driver. This mostly works, but has a few problems:

1. It relies on the application author passing char** argv to the QApplication constructor. This happens less often than you might think.

1. We want to load the testability driver recursively. For example, if autopilot launches 'app_a' with -testability, and this application launches another Qt app, we want that to load with testability as well. Currently this requires that the 'app_a' author codes this behavior in.

1. Some Ubuntu Touch applications are going to be launched by upstart jobs. This makes it harder to set, or remove the -testability command line option.

Proposed Solution

I propose we check an environment variable QT_LOAD_TESTABILITY. If it exists and is set to something other than an empty string, we should load the testability library.

Attached is a three line patch that, when applied to src/widgets/kernel/qapplication.cpp adds this functionality.