tests: add functional tests for DB message persistence, rebalance groups

Add backend-agnostic functional tests for database message persistence.
All tests work with both SQLite and flat-file backends.

New tests in test_history.c:
- message_db_history_on_reopen: basic incoming write+read round-trip
- message_db_history_multiple: 3 messages from different resources
- message_db_history_contact_isolation: buddy1 msg absent from buddy2
- message_db_history_special_chars: XML entities survive decode+DB
- message_db_history_outgoing: sent message persists across reopen
- message_db_history_dialog: outgoing + incoming both in history
- message_db_history_empty: no crash on contact with no history
- message_db_history_long_message: 1000+ char body not truncated
- message_db_history_newline: embedded LF stored correctly
- message_db_history_service_chars: backslash pipe percent braces etc
- message_db_history_verify: /history verify (ifdef HAVE_HISTORY_VERIFY)
- message_db_history_lmc: XEP-0308 correction replaces original in DB

Rebalance test groups for parallel execution (19/22/22/21):
- Group 1: Connect, Ping, Rooms, Software, LastActivity
- Group 2: Message, Receipts, Roster, DB History
- Group 3: Chat Session, Presence, Disconnect
- Group 4: MUC, Carbons

Makefile.am

@@ -189,6 +189,7 @@ functionaltest_sources = \
 	tests/functionaltests/test_software.c tests/functionaltests/test_software.h \
 	tests/functionaltests/test_muc.c tests/functionaltests/test_muc.h \
 	tests/functionaltests/test_disconnect.c tests/functionaltests/test_disconnect.h \
+	tests/functionaltests/test_history.c tests/functionaltests/test_history.h \
 	tests/functionaltests/test_lastactivity.c tests/functionaltests/test_lastactivity.h \
 	tests/functionaltests/test_autoping.c tests/functionaltests/test_autoping.h \
 	tests/functionaltests/test_disco.c tests/functionaltests/test_disco.h \