tests: test vars generation for all clan service tests

pkgs/testing/flake-module.nix

@@ -0,0 +1,18 @@
+{
+  perSystem = {
+    legacyPackages.setupNixInNix = ''
+      export HOME=$TMPDIR
+      export NIX_STATE_DIR=$TMPDIR/nix
+      export IN_NIX_SANDBOX=1
+      export CLAN_TEST_STORE=$TMPDIR/store
+      # required to prevent concurrent 'nix flake lock' operations
+      export LOCK_NIX=$TMPDIR/nix_lock
+      mkdir -p "$CLAN_TEST_STORE/nix/store"
+      mkdir -p "$CLAN_TEST_STORE/nix/var/nix/gcroots"
+      if [[ -n "''${closureInfo-}" ]]; then
+        xargs cp --recursive --target "$CLAN_TEST_STORE/nix/store"  < "$closureInfo/store-paths"
+        nix-store --load-db --store "$CLAN_TEST_STORE" < "$closureInfo/registration"
+      fi
+    '';
+  };
+}