fix(settings): drop Test Dictation scrolled-window stripe (#853)

* fix(settings): drop Test Dictation scrolled-window stripe

GTK3 ScrolledWindow defaults to an IN etched shadow, which GNOME/Wayland
renders as a thin vertical stripe on the left of the Test Dictation
revealer even when the buffer is empty. Call set_shadow_type(NONE) to
match the other ScrolledWindows in this dialog.

Fixes #847

* Add missing test return annotation

src/vocalinux/ui/settings_dialog.py

@@ -4055,6 +4055,8 @@ class SettingsDialog(Gtk.Dialog):
         scrolled_window.set_min_content_height(60)
         scrolled_window.set_max_content_height(100)
         scrolled_window.set_policy(Gtk.PolicyType.AUTOMATIC, Gtk.PolicyType.AUTOMATIC)
+        # GTK3 default IN shadow draws a left etched stripe on GNOME/Wayland (#847).
+        scrolled_window.set_shadow_type(Gtk.ShadowType.NONE)
         scrolled_window.get_style_context().add_class("test-area")
 
         self.test_textview = Gtk.TextView()