Instead of using an environment variable, pass the path to the config
file as a command line argument. This makes it possible to directly
copy the command being run (as show with --verbose) and run it in a
debugger outside of the test framework.
Old verbose output:
Running: /home/ajor/src/object-introspection3/build/test/integration/integration_test_target oil cycles_unique_ptr
New verbose output:
Running: /home/ajor/src/object-introspection3/build/test/integration/integration_test_target oil cycles_unique_ptr /home/ajor/src/object-introspection3/build/testing.oid.toml
