tests: adjust disabling donation prompt, yet again

The mechanism to disable it changed again...
This affects at least the version in Debian 12.

Author: marmarek

tests/splitgpg/tests.py

@@ -505,6 +505,7 @@ def setup_tb_profile(self, setup_openpgp):
 user_pref("mail.identity.id1.compose_html", false);
 user_pref("datareporting.policy.dataSubmissionEnabled", false); // avoid message popups
 user_pref("app.donation.eoy.version.viewed", 100); // avoid message popups
+user_pref("mail.inappnotifications.enabled", false); // avoid message popups
 """
         imap_server = """
 user_pref("mail.server.server1.userName", "user");