tests: Verify GParamSpecPool creation

Ensure that the fix in commit af024b6d7e7d3fbef23c1f7d1f7704fc90ac4fb1
works, by replicating what gobject-introspection does:

- initialise the default GTypeInterface from a GType without also
  initialising GObjectClass
  - install a property for the interface
- find the GParamSpec using g_object_interface_find_property()

gobject/tests/meson.build

@@ -83,6 +83,7 @@ gobject_tests = {
   'binding' : {},
   'bindinggroup' : {},
   'properties' : {},
+  'properties-introspection' : {},
   'reference' : {},
   'flags' : {},
   'value' : {},

gobject/tests/properties-introspection.c

@@ -0,0 +1,60 @@
+/* properties-introspection.c: Test the properties introspection API
+ *
+ * SPDX-FileCopyrightText: 2023 Emmanuele Bassi
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ */
+
+/* This test is isolated so we can control the initialization of
+ * GObjectClass, and the global GParamSpecPool
+ */
+
+#include <stdlib.h>
+#include <glib-object.h>
+
+G_DECLARE_INTERFACE (MyTestable, my_testable, MY, TESTABLE, GObject)
+
+struct _MyTestableInterface
+{
+  GTypeInterface g_iface;
+};
+
+G_DEFINE_INTERFACE (MyTestable, my_testable, G_TYPE_OBJECT)
+
+static void
+my_testable_default_init (MyTestableInterface *iface)
+{
+  g_object_interface_install_property (iface,
+    g_param_spec_int ("check", NULL, NULL, -1, 10, 0, G_PARAM_READWRITE));
+}
+
+static void
+properties_introspection (void)
+{
+  g_test_summary ("Verify that introspecting properties on an interface initializes the GParamSpecPool.");
+
+  if (g_test_subprocess ())
+    {
+      gpointer klass = g_type_default_interface_ref (my_testable_get_type ());
+      g_assert_nonnull (klass);
+
+      GParamSpec *pspec = g_object_interface_find_property (klass, "check");
+      g_assert_nonnull (pspec);
+
+      g_type_default_interface_unref (klass);
+      return;
+    }
+
+  g_test_trap_subprocess (NULL, 0, G_TEST_SUBPROCESS_DEFAULT);
+  g_test_trap_assert_passed ();
+  g_test_trap_assert_stderr ("");
+}
+
+int
+main (int argc, char *argv[])
+{
+  g_test_init (&argc, &argv, NULL);
+
+  g_test_add_func ("/properties/introspection", properties_introspection);
+
+  return g_test_run ();
+}