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/test_export_import.h

@@ -0,0 +1,2 @@
+void export_sqlite_to_flatfile(void** state);
+void import_flatfile_to_sqlite(void** state);