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)

0 files changed, 0 insertions, 0 deletions