test: Rename early key response test and move random_bitflip to util

Early key response test is a special kind of test which requires
modified v2 handshake functions. More such tests can be added
where v2 handshake functions send incorrect garbage terminator,
excess garbage bytes etc.. Hence, rename p2p_v2_earlykey.py to a
general test file name - p2p_v2_misbehaving.py.

random_bitflip function (used in signature tests prior to this
commit) can be used in p2p_v2_misbehaving test to generate wrong
garbage terminator, wrong garbage bytes etc..
So, move the function to util.

test/functional/test_framework/key.py

@@ -14,6 +14,7 @@ import random
 import unittest
 
 from test_framework.crypto import secp256k1
+from test_framework.util import random_bitflip
 
 # Point with no known discrete log.
 H_POINT = "50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0"
@@ -292,11 +293,6 @@ def sign_schnorr(key, msg, aux=None, flip_p=False, flip_r=False):
 class TestFrameworkKey(unittest.TestCase):
     def test_ecdsa_and_schnorr(self):
         """Test the Python ECDSA and Schnorr implementations."""
-        def random_bitflip(sig):
-            sig = list(sig)
-            sig[random.randrange(len(sig))] ^= (1 << (random.randrange(8)))
-            return bytes(sig)
-
         byte_arrays = [generate_privkey() for _ in range(3)] + [v.to_bytes(32, 'big') for v in [0, ORDER - 1, ORDER, 2**256 - 1]]
         keys = {}
         for privkey_bytes in byte_arrays:  # build array of key/pubkey pairs