[gtksourceview] tests: fix initialization with gtk4

[Christian Hergert <chergert src gnome org>]

commit b3420d91c2c0bd1305c68820b17923b2e977924e
Author: Christian Hergert <chergert redhat com>
Date:   Thu Nov 19 10:00:50 2020 -0800

    tests: fix initialization with gtk4
    
    This will get fixed in GTK 4 eventually, but allows us to run things now.

 tests/test-widget.c | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/tests/test-widget.c b/tests/test-widget.c
index bb1b3f1b..6824dd3d 100644
--- a/tests/test-widget.c
+++ b/tests/test-widget.c
@@ -1173,6 +1173,8 @@ main (int argc, char *argv[])
                cmd_filename = argv[1];
        }
 
+       g_set_prgname ("test-widget");
+
        gtk_init ();
        gtk_source_init ();
        setup_search_paths ();