The context is easier to reason about: E.g.,
* in TestBasicMining it allows to drop manual SetMockTime(0) calls,
* in connections_desirable_service_flags it allows to drop manual calls
to SetMockTime(GetTime<std::chrono::seconds>() + _n_) and replace them
by operator+=(_n_)
* in wallet_tests it clarifies that the mocktime does not persist
outside the AddTx function