Prepare MariaDB restore tests and recover backup disk space safely

lib/report.py

@@ -27,7 +27,10 @@ def allocated(path):
 
 def snapshot(output, staging, archives):
     data = {'staged': {}, 'archives': {}}
-    if staging and Path(staging).is_dir():
+    if Path(output).is_file():
+        data.update(json.loads(Path(output).read_text()))
+    captured = 'staging_total' in data
+    if not captured and staging and Path(staging).is_dir():
         root = Path(staging)
         # Measure components independently; the total accounts for shared links.
         paths = sorted((root/'files').glob('*')) + [root/'db', root/'meta']