Add functional and unit tests for DB export/import

- 2 functional tests: export_sqlite_to_flatfile, import_flatfile_to_sqlite
  with XEP-0203 delay timestamps for deterministic verification
- 27 unit tests for database_export covering edge cases
- Merge test/db-functional-tests: 12 DB persistence tests (test_history)
- Group 2 rebalanced: 25 tests (23 base + 2 SQLite-only export/import)
- #ifdef HAVE_SQLITE guards for export/import tests
- prof_stop() helper in proftest.c
- Makefile.am: source ordering cleanup, new test files added

tests/functionaltests/proftest.h

@@ -15,6 +15,7 @@ int init_prof_test(void **state);
 int close_prof_test(void **state);
 
 void prof_start(void);
+void prof_stop(void);
 void prof_connect(void);
 void prof_connect_with_roster(const char *roster);
 void prof_input(const char *input);