[M0/Mx] Fix test_route_flap (sonic-net#15641)

PR sonic-net#14804 caused test_route_flap fail on M0/Mx with below error:

>       pytest.fail("Did not find a dut in duthosts that for topo type {} that has upstream nbr type {}".
                    format(tbinfo["topo"]["type"], upstream_nbr_type))
E       Failed: Did not find a dut in duthosts that for topo type m0 that has upstream nbr type T3

What is the motivation for this PR?
Fix test_route_flap on M0/Mx topo.

How did you do it?
Support upstream neighbor on M0/Mx.

How did you verify/test it?
Verified on Nokia-7215 M0.
Verified on Arista-720DT M0.

tests/conftest.py

@@ -1875,7 +1875,11 @@ def enum_rand_one_frontend_asic_index(request):
 
 @pytest.fixture(scope='module')
 def enum_upstream_dut_hostname(duthosts, tbinfo):
-    if tbinfo["topo"]["type"] == "t0":
+    if tbinfo["topo"]["type"] == "m0":
+        upstream_nbr_type = "M1"
+    elif tbinfo["topo"]["type"] == "mx":
+        upstream_nbr_type = "M0"
+    elif tbinfo["topo"]["type"] == "t0":
         upstream_nbr_type = "T1"
     elif tbinfo["topo"]["type"] == "t1":
         upstream_nbr_type = "T2"