tests/functional: support loading (contributed) plugins

This is useful to test extended features in Lix.

Change-Id: Idb2416a080329116677809b883950e6c33028a44
Signed-off-by: Raito Bezarius <raito@lix.systems>

tests/functional/common/vars-and-functions.sh.in

@@ -251,6 +251,21 @@ runinpty() {
     @python@ @runinpty@ "$@"
 }
 
+# Add a plugin to `plugin-files` configuration.
+loadPlugin() {
+    local plugin_path="$1"
+    sed -i 's|plugin-files .*|& '"$plugin_path"'|' "$NIX_CONF_DIR"/nix.conf
+}
+
+loadContribPlugin() {
+    local plugin_name="$1"
+    libext=so
+    if [[ "$(uname -s)" == Darwin ]]; then
+        libext=dylib
+    fi
+    loadPlugin "$MESON_BUILD_ROOT/contrib/plugins/libplugin_$plugin_name.$libext"
+}
+
 set -x
 
 onError() {

tests/functional/init.sh

@@ -27,6 +27,7 @@ gc-reserved-space = 0
 substituters =
 flake-registry = $TEST_ROOT/registry.json
 show-trace = true
+plugin-files =
 include nix.conf.extra
 trusted-users = $(whoami || id -u)
 EOF