This test-only change allows python (rpc) tests to specify, for example,
that NU5 should be activated at height X, without having to specify all
the previous network upgrades. Previous upgrades can (and must) still be
specified if they activate at different block heights (than, in this
example, NU5). This makes tests easier to write (and read), especially
as the number of network upgrades increases over time.
Note that this change only affects zcashd behavior in regtest mode. For
the other network modes (testnet and mainnet), the activation heights
are hard-coded in chainparams.cpp.
Larry Ruane