test: fix assert_debug_log call-site bugs, add type checks

Two recently added tests (PR #28625 / commit 2e31250027
and PR #28634 / commit 3bb51c29df)
introduced a bug by wrongly using the `assert_debug_log` helper.
Instead of passing the expected debug string in a list as expected, it
was passed as bare string, which is then interpretered as a list of
characters, very likely leading the debug log assertion pass even if the
intended message is not appearing.

In order to avoid bugs like this in the future, enforce that the
`{un}expected_msgs` parameters are lists.

test/functional/p2p_v2_transport.py

@@ -145,7 +145,7 @@ class V2TransportTest(BitcoinTestFramework):
         wrong_network_magic_prefix = MAGIC_BYTES["signet"] + V1_PREFIX[4:]
         with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
             s.connect(("127.0.0.1", p2p_port(0)))
-            with self.nodes[0].assert_debug_log("V2 transport error: V1 peer with wrong MessageStart"):
+            with self.nodes[0].assert_debug_log(["V2 transport error: V1 peer with wrong MessageStart"]):
                 s.sendall(wrong_network_magic_prefix + b"somepayload")
 
         # Check detection of missing garbage terminator (hits after fixed amount of data if terminator never matches garbage)
@@ -156,7 +156,7 @@ class V2TransportTest(BitcoinTestFramework):
             self.wait_until(lambda: len(self.nodes[0].getpeerinfo()) == num_peers + 1)
             s.sendall(b'\x00' * (MAX_KEY_GARB_AND_GARBTERM_LEN - 1))
             self.wait_until(lambda: self.nodes[0].getpeerinfo()[-1]["bytesrecv"] == MAX_KEY_GARB_AND_GARBTERM_LEN - 1)
-            with self.nodes[0].assert_debug_log("V2 transport error: missing garbage terminator"):
+            with self.nodes[0].assert_debug_log(["V2 transport error: missing garbage terminator"]):
                 s.sendall(b'\x00')  # send out last byte
                 # should disconnect immediately
                 self.wait_until(lambda: len(self.nodes[0].getpeerinfo()) == num_peers)