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/examples/bat.conf

@@ -1,5 +1,5 @@
 # Make sure that the pager gets executed
 --paging=always
 
-# Output a dummy message for the integration test.
+# Output a dummy message for the integration test and system wide config test.
 --pager="echo dummy-pager-from-config"

tests/examples/system_config/bat/config

@@ -0,0 +1,5 @@
+# Make sure that the pager gets executed
+--paging=always
+
+# Output a dummy message for the integration test.
+--pager="echo dummy-pager-from-system-config"