test: improve rpc_blockchain.py tests and assert on time and mediantime

Summary:
Backport of core#22520.

Depends on D12551.

Test Plan:

./test/functional/test_runner.py rpc_blockchain

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12552