tests: Add a script to run the tests in a LD_LIBRARY_PATH environment
This should not be expected to pass reliably yet, particularly if the version of steam-runtime-tools included in the LD_LIBRARY_PATH runtime is not in sync with the version that provided the tests.
This doesn't use ginsttest-runner and its .test files, for two reasons:
- the gnome-desktop-testing package isn't included in the LD_LIBRARY_PATH variant of the Steam Runtime;
- the .test files are not relocatable (they name tests according to their absolute path), but to test a relocatable LD_LIBRARY_PATH runtime, we need relocatable tests