loaders.tests: Simplify and add coverage to check_snapshot

The uncovered conditional is dead code since tests no longer run with in-memory
storage.

This adds the necessary scenario to check the no snapshot found case is
covered.

Related to T2486

swh/loader/tests/__init__.py

@@ -116,20 +116,7 @@ def check_snapshot(expected_snapshot, storage):
     expected_branches = expected_snapshot["branches"]
     snap = storage.snapshot_get(hash_to_bytes(expected_snapshot_id))
     if snap is None:
-        # display known snapshots instead if possible
-        if hasattr(storage, "_snapshots"):  # in-mem storage
-            from pprint import pprint
-
-            for snap_id, (_snap, _) in storage._snapshots.items():
-                snapd = _snap.to_dict()
-                snapd["id"] = hash_to_hex(snapd["id"])
-                branches = {
-                    branch.decode("utf-8"): decode_target(target)
-                    for branch, target in snapd["branches"].items()
-                }
-                snapd["branches"] = branches
-                pprint(snapd)
-        raise AssertionError("Snapshot is not found")
+        raise AssertionError(f"Snapshot {expected_snapshot_id} is not found")
 
     branches = {
         branch.decode("utf-8"): decode_target(target)

swh/loader/tests/test_init.py

@@ -243,11 +243,16 @@ def test_check_snapshot_failure():
     }
 
     unexpected_snapshot = {
-        "id": "2498dbf535f882bc7f9a18fb16c9ad27fda7bab7",
+        "id": "2498dbf535f882bc7f9a18fb16c9ad27fda7bab7",  # id is correct
         "branches": {
-            "master": {"target": hash_hex, "target_type": "release",}  # wrong value
+            "master": {"target": hash_hex, "target_type": "release",}  # wrong branch
         },
     }
 
-    with pytest.raises(AssertionError):
+    with pytest.raises(AssertionError, match="Differing items"):
+        check_snapshot(unexpected_snapshot, storage)
+
+    # snapshot id which does not exist
+    unexpected_snapshot["id"] = "999666f535f882bc7f9a18fb16c9ad27fda7bab7"
+    with pytest.raises(AssertionError, match="is not found"):
         check_snapshot(unexpected_snapshot, storage)