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.c

@@ -436,6 +436,24 @@ close_prof_test(void **state)
     return 0;
 }
 
+void
+prof_stop(void)
+{
+    if (fd > 0 && child_pid > 0) {
+        prof_input("/quit");
+        sleep(1);
+        waitpid(child_pid, NULL, 0);
+        close(fd);
+        fd = 0;
+        child_pid = 0;
+    }
+    /* Reset output buffer for clean restart */
+    output_len = 0;
+    output_buffer[0] = '\0';
+    /* Brief delay to let stabber release old connection state */
+    usleep(200000);  /* 200ms */
+}
+
 void
 prof_input(const char *input)
 {