extra: move locale back to top-level

Avoid extra/extra. Add basic test.

Author: zimbatm

extra/extra/locale.nix extra/locale.nixextra/extra/locale.nix renamed to extra/locale.nix

@@ -0,0 +1,21 @@
+{ pkgs, devshell, runTest }:
+{
+  # Basic test
+  simple =
+    let
+      shell = devshell.mkShell {
+        imports = [ ../../extra/locale.nix ];
+        devshell.name = "locale-simple";
+      };
+    in
+    runTest "simple" { } ''
+      # Assume that LOCAL_ARCHIVE is not set before
+      assert -z "$LOCALE_ARCHIVE"
+
+      # Load the devshell
+      source ${shell}
+
+      # Sets LOCALE_ARCHIVE
+      assert -n "$LOCALE_ARCHIVE"
+    '';
+}