Add functional tests for include directive in nix config file

tests/functional/config.sh

@@ -43,6 +43,16 @@ export NIX_USER_CONF_FILES=$here/config/nix-with-substituters.conf
 var=$(nix config show | grep '^substituters =' | cut -d '=' -f 2 | xargs)
 [[ $var == https://example.com ]]
 
+# Test that we can include a file.
+export NIX_USER_CONF_FILES=$here/config/nix-with-include.conf
+var=$(nix config show | grep '^allowed-uris =' | cut -d '=' -f 2 | xargs)
+[[ $var == https://github.com/NixOS/nix ]]
+
+# Test that we can !include a file.
+export NIX_USER_CONF_FILES=$here/config/nix-with-bang-include.conf
+var=$(nix config show | grep '^experimental-features =' | cut -d '=' -f 2 | xargs)
+[[ $var == nix-command ]]
+
 # Test that it's possible to load config from the environment
 prev=$(nix config show | grep '^cores' | cut -d '=' -f 2 | xargs)
 export NIX_CONFIG="cores = 4242"$'\n'"experimental-features = nix-command flakes"
@@ -56,4 +66,4 @@ exp_features=$(nix config show | grep '^experimental-features' | cut -d '=' -f 2
 # Test that it's possible to retrieve a single setting's value
 val=$(nix config show | grep '^warn-dirty' | cut -d '=' -f  2 | xargs)
 val2=$(nix config show warn-dirty)
-[[ $val == $val2 ]]
+[[ $val == $val2 ]]

tests/functional/config/extra-config.conf

@@ -0,0 +1 @@
+allowed-uris = https://github.com/NixOS/nix

tests/functional/config/nix-with-bang-include.conf

@@ -0,0 +1,2 @@
+experimental-features = nix-command
+!include ./missing-extra-config.conf

tests/functional/config/nix-with-include.conf

@@ -0,0 +1,2 @@
+experimental-features = nix-command
+include ./extra-config.conf