Suppress shared test helper warnings

Keep the downgrade test focused by allowing unused imports and macros
from the shared integration-test helper module it includes.

Co-Authored-By: HAL 9000

Author: tnull

tests/upgrade_downgrade_tests.rs

@@ -16,6 +16,7 @@
 // the legacy v1 filesystem-store implementation via `build_with_store`, then
 // reopen it with v0.7.0's `build_with_fs_store`.
 
+#[allow(unused_imports, unused_macros)]
 mod common;
 
 use std::path::PathBuf;