Fix script used to run libardour tests
Add get_test_sample_rate to common test functions in libardour tests
Delete test_globals.h/cc now that the contents has moved to test_common.h/cc
Update test to use correct SourceFactory::createWritable API
Use test_search_path to find test data files for automation test
return bool in test util function to check writing xml to file is successful
Add arg to test function to be able to prefix test output directory

This makes it much easier to find test output from a specific test
Write state data from automation list test to check why test is failing
Use test-env.sh in run-session-tests.sh

Also make the script executable
Add AudioEngine::set_default_backend
Fix ardour tests to work with new AudioEngine/Backends changes
don't use a possibly non-existent window as a transient parent when showing a dialog about engine failure