Improve the getblock test in rpc_blockchain

Summary: This diff removes the need for both the txindex and the wallet from the _test_getblock test. This will make it possible to run the test with no wallet in a later backport. Note that this test case is ABC only.

Test Plan:

./test/functional/test_runner.py rpc_blockchain

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12444