[backport#16865] test: add some unit tests for merkle.cpp

Summary:
following situations are covered:

• empty block
• one Tx
• Merkle root of a block with odd Txs should not change with repeating

last one

• Merkle root is computed with combining Merkle root of left subtree and right subtree

Signed-off-by: soroosh-sdi <soroosh.sardari@gmail.com>

Backport of Core PR16865

Test Plan: ninja && ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8017