Fixed bug in test_shared_memory.py (#1128)

tests/vector/utils/test_shared_memory.py

@@ -23,11 +23,13 @@ from tests.spaces.utils import TESTING_SPACES, TESTING_SPACES_IDS
 )
 def test_shared_memory_create_read_write(space, num, ctx):
     """Test the shared memory functions, create, read and write for all testing spaces."""
-    ctx = mp if ctx is None else mp.get_context(ctx)
-    if ctx not in mp.get_all_start_methods():
+    try:
+        _ctx = mp.get_context(ctx)
+    except ValueError:
         pytest.skip(
             f"Multiprocessing start method {ctx} not available on this platform."
         )
+    ctx = _ctx
 
     samples = [space.sample() for _ in range(num)]