Pass arbitrary keyword args to load_test_app

Applications can be written with arbitrary keyword arguments
in `setup_app`. The test app builder should support this by
allowing arbitrary keyword args to be passed to `load_test_app`,
and pass those along to `load_app`.

Change-Id: Icb3d5c39d0bf147c1e58732be3199a30a7539f6f
Merge "Pass arbitrary keyword args to load_test_app"