Make APF packet count assertion more robust

Improve the APF test by changing the packet count assertion from
`== 1` to `> matched_pkt_count_before_test`.

This handles scenarios where the packet counter might already
have a non-zero value or increment due to other traffic, making
the test more reliable. The test now asserts that the specific
test packet caused the counter to increase.

Flag: EXEMPT test only
Bug: 419427388
Test: atest CtsConnectivityMultiDevicesTestCases:ApfV6Test
Change-Id: Id3c0de786e674214948e7030519fe25383c9958c

staticlibs/testutils/host/python/apf_test_base.py

@@ -140,6 +140,10 @@ class ApfTestBase(multi_devices_test_base.MultiDevicesTestBase):
           counter_name,
       )
 
+      matched_pkt_count_before_test = apf_utils.get_matched_packet_counts(
+          self.serverDevice, self.server_iface_name, receive_packet
+      )
+
       # send 3 packets to prevent flaky test result
       for _ in range(3):
         apf_utils.send_raw_packet_downstream(
@@ -150,7 +154,7 @@ class ApfTestBase(multi_devices_test_base.MultiDevicesTestBase):
           lambda: apf_utils.get_matched_packet_counts(
               self.serverDevice, self.server_iface_name, receive_packet
           )
-          == 1,
+          > matched_pkt_count_before_test,
           # ensure the server device capturing the offload packet on the handler thread
           retry_interval_sec=3,
       )