test: added fuzz coverage for consensus/merkle.cpp

[kevkevinpal]

mentioned in: bitcoin#32243

bitcoin#32243 (comment)

"""
Approach looks good to me and this test also adds coverage to line we seem to miss in non-fuzz tests

Missing line in unit and functional tests coverage

Got 100 % coverage for the limited time I ran this fuzz test.
""" - Prabhat1308

[kevkevinpal]

Will be closed by this change bitcoin#33858