[template-glib] tests: add error log before failing

[Christian Hergert <chergert src gnome org>]

commit 11186cbb1de24005ba5b9578d3f5489885b7a3f6
Author: Christian Hergert <chergert redhat com>
Date:   Wed May 4 17:43:51 2022 -0700

    tests: add error log before failing
    
    Just to make quick testing as we fix things easier.

 tests/test-expr.c | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/tests/test-expr.c b/tests/test-expr.c
index 0268078..18d85fd 100644
--- a/tests/test-expr.c
+++ b/tests/test-expr.c
@@ -53,6 +53,10 @@ test1 (void)
   g_assert_no_error (error);
   g_assert_true (r);
 
+  if (!G_VALUE_HOLDS_DOUBLE (&ret))
+    g_printerr ("Expected double, got %s\n",
+                G_VALUE_TYPE_NAME (&ret));
+
   g_assert_true (G_VALUE_HOLDS_DOUBLE (&ret));
   g_assert_cmpint (g_value_get_double (&ret), ==, 1234.0);