feat: Don't fetch messages from unknown folders (#7190)

Actually this leads to fetching messages only from watched folders and Spam. Motivation:
- At least Gmail has virtual folders which aren't correctly detected as such, e.g. "Sent".
- At least Gmail has many virtual folders and scanning all of them takes significant time, 5-6 secs
  in median for me. This slows down receiving new messages and consumes battery.
- Delta Chat shouldn't fetch messages from folders potentially created by other apps for their own
  purposes. NB: All compatible Delta Chat forks should use the "DeltaChat" folder as mvbox.
- Fetching from folders that aren't watched, e.g. from "Sent", may lead to message ordering issues.

python/tests/test_1_online.py

@@ -1755,12 +1755,12 @@ def test_group_quote(acfactory, lp):
             "xyz",
             False,
             "xyz",
-        ),  # Test that emails are recognized in a random folder but not moved
+        ),  # Test that emails aren't found in a random folder
         (
-            "xyz",
+            "Spam",
             True,
             "DeltaChat",
-        ),  # ...emails are found in a random folder and moved to DeltaChat
+        ),  # ...emails are moved from the spam folder to "DeltaChat"
         (
             "Spam",
             False,
@@ -1785,7 +1785,7 @@ def test_scan_folders(acfactory, lp, folder, move, expected_destination):
     ac1.stop_io()
     assert folder in ac1.direct_imap.list_folders()
 
-    lp.sec("Send a message to from ac2 to ac1 and manually move it to the mvbox")
+    lp.sec("Send a message to from ac2 to ac1 and manually move it to `folder`")
     ac1.direct_imap.select_config_folder("inbox")
     with ac1.direct_imap.idle() as idle1:
         acfactory.get_accepted_chat(ac2, ac1).send_text("hello")
@@ -1795,10 +1795,17 @@ def test_scan_folders(acfactory, lp, folder, move, expected_destination):
     lp.sec("start_io() and see if DeltaChat finds the message (" + variant + ")")
     ac1.set_config("scan_all_folders_debounce_secs", "0")
     ac1.start_io()
-    msg = ac1._evtracker.wait_next_incoming_message()
-    assert msg.text == "hello"
+    chat = ac1.create_chat(ac2)
+    n_msgs = 1  # "Messages are end-to-end encrypted."
+    if folder == "Spam":
+        msg = ac1._evtracker.wait_next_incoming_message()
+        assert msg.text == "hello"
+        n_msgs += 1
+    else:
+        ac1._evtracker.wait_idle_inbox_ready()
+    assert len(chat.get_messages()) == n_msgs
 
-    # The message has been downloaded, which means it has reached its destination.
+    # The message has reached its destination.
     ac1.direct_imap.select_folder(expected_destination)
     assert len(ac1.direct_imap.get_all_messages()) == 1
     if folder != expected_destination: