graph: add test for the "algo" parameter of walk()

If I change the code like this:

diff --git a/swh/graph/backend.py b/swh/graph/backend.py
index 750849a..7facd5c 100644
--- a/swh/graph/backend.py
+++ b/swh/graph/backend.py
@@ -75,6 +75,7 @@ def count(self, ttype, direction, edges_fmt, src):
             yield node_id
 
     async def walk(self, direction, edges_fmt, algo, src, dst):
+        algo = "bfs"
         if dst in EXTENDED_SWHID_TYPES:
             it = self.stream_proxy.walk_type(direction, edges_fmt, algo, src, dst)
         else:

Then it breaks no test. Likewise with "dfs" instead of "bfs".

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Migrated from T3301 (view on Phabricator)