test: Use msg_generic in p2p_ping.py #32834

It seems odd to derive msg_pong_corrupt from msg_pong, but then overwrite the serialize method, when one can just directly use msg_generic to pass the raw bytes to send over the wire.

Fix that by using msg_generic. This also serves as a regression test against the fix in commit 33480573cbd8d03aefbde100e51f827a2f7de7f7.

(Can be tested by reverting that commit to observe a failure)

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/32834.

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utACK fa3f100010f1a0273891ed0d14ddb3c5ccacbd4b

ACK fa3f100010f1a0273891ed0d14ddb3c5ccacbd4b

Checked that this catches the issue fixed in #32833 as expected (i.e. p2p_ping.py fails if the msgtype slot is removed again from msg_generic).