Add test for systemwide config file support

There is now a new stage in the CICD workflow present, which will build
`bat` with the `BAT_SYSTEM_CONFIG_PREFIX` set to load the config file
from `/tests/examples/system_config/bat/config`, plus a basic set of
tests, to ensure the feature is working as expected. By default the
tests are set to ignored, as they need special setup before they can be
run.

tests/utils/mod.rs

@@ -1 +1,2 @@
+pub mod command;
 pub mod mocked_pagers;