tests: add a dependency on generated headers
This fixes the following random errors:
../gnome-control-center-3.32.1/tests/datetime/test-timezone-gfx.c:5:10: fatal error: 'cc-datetime-resources.h' file not found
../gnome-control-center-3.32.1/tests/interactive-panels/main.c:29:10: fatal error: 'shell/resources.h' file not found