Small improvements to the Taproot functional tests

The "whitelist" and "connect_nodes" is not needed in feature_taproot.py,
so remove it.

The changes to key.py are required when running the unit tests from the
test folder. Failure on current master:

[test]$ python -m unittest functional/test_framework/key.py
.E
======================================================================
ERROR: test_schnorr_testvectors (functional.test_framework.key.TestFrameworkKey)
Implement the BIP340 test vectors (read from bip340_test_vectors.csv).
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test/functional/test_framework/key.py", line 526, in test_schnorr_testvectors
    with open(os.path.join(sys.path[0], 'test_framework', 'bip340_test_vectors.csv'), newline='', encoding='utf8') as csvfile:
FileNotFoundError: [Errno 2] No such file or directory: 'test/test_framework/bip340_test_vectors.csv'

----------------------------------------------------------------------
Ran 2 tests in 0.775s

FAILED (errors=1)

Author: sipa

test/functional/feature_taproot.py

@@ -1200,7 +1200,7 @@ def set_test_params(self):
         self.num_nodes = 2
         self.setup_clean_chain = True
         # Node 0 has Taproot inactive, Node 1 active.
-        self.extra_args = [["-whitelist=127.0.0.1", "-par=1", "-vbparams=taproot:1:1"], ["-whitelist=127.0.0.1", "-par=1"]]
+        self.extra_args = [["-par=1", "-vbparams=taproot:1:1"], ["-par=1"]]
 
     def block_submit(self, node, txs, msg, err_msg, cb_pubkey=None, fees=0, sigops_weight=0, witness=False, accept=False):
 
@@ -1437,8 +1437,6 @@ def test_spenders(self, node, spenders, input_counts):
         self.log.info("  - Done")
 
     def run_test(self):
-        self.connect_nodes(0, 1)
-
         # Post-taproot activation tests go first (pre-taproot tests' blocks are invalid post-taproot).
         self.log.info("Post-activation tests...")
         self.nodes[1].generate(101)

test/functional/test_framework/key.py

@@ -10,7 +10,6 @@
 import hashlib
 import os
 import random
-import sys
 import unittest
 
 from .util import modinv
@@ -22,6 +21,7 @@ def TaggedHash(tag, data):
     return hashlib.sha256(ss).digest()
 
 def xor_bytes(b0, b1):
+    assert len(b0) == len(b1)
     return bytes(x ^ y for (x, y) in zip(b0, b1))
 
 def jacobi_symbol(n, k):
@@ -523,7 +523,8 @@ def test_schnorr(self):
     def test_schnorr_testvectors(self):
         """Implement the BIP340 test vectors (read from bip340_test_vectors.csv)."""
         num_tests = 0
-        with open(os.path.join(sys.path[0], 'test_framework', 'bip340_test_vectors.csv'), newline='', encoding='utf8') as csvfile:
+        vectors_file = os.path.join(os.path.dirname(os.path.realpath(__file__)), 'bip340_test_vectors.csv')
+        with open(vectors_file, newline='', encoding='utf8') as csvfile:
             reader = csv.reader(csvfile)
             next(reader)
             for row in reader: