test-client: Fix test failure for new composer widget tests under CI

Init icon factory so composer tests work under CI, add rsvg as a test
dependency for Ubuntu which as of 20.10 does not include it by default.

test/test-client.vala

@@ -32,6 +32,7 @@ int main(string[] args) {
     Gtk.init(ref args);
     Test.init(ref args);
 
+    IconFactory.init(GLib.File.new_for_path(_SOURCE_ROOT_DIR));
     Geary.RFC822.init();
     Geary.HTML.init();
     Geary.Logging.init();