Add missing import

Was causing the test to fail. It's annoying that you don't get an error
from that earlier..

test_runner/regress/test_remote_storage.py

@@ -12,6 +12,7 @@ from fixtures.log_helper import log
 from fixtures.neon_fixtures import (
     NeonEnvBuilder,
     RemoteStorageKind,
+    assert_no_in_progress_downloads_for_tenant,
     available_remote_storages,
     wait_for_last_record_lsn,
     wait_for_last_flush_lsn,