mining: use interface for tests, bench and fuzzers

Have most tests, benchmarks and fuzzers go through the mining interface.
This is a refactor: it does not change what blocks are created, just how
the creation calls are made. This avoids most direct test, benchmark and
fuzzer use of node::BlockAssembler::Options, making it easier to drop in a
later commit.

Two exceptions which use BlockAssembler directly:
- one check in test/miner_tests.cpp needs m_package_feerates
- fuzz/tx_pool.cpp Finish() doesn't have access to a NodeContext

Move test_block_validity from BlockAssembler::Options to
BlockCreateOptions so bench/block_assemble.cpp can continue to set it.
Just like coinbase_output_script, this is not exposed to IPC clients.

Inline options variable in places where it's only needed once.

We also drop one unused PrepareBlock declaration and one unused
implementation.

TestChain100Setup::CreateBlock no longer needs a chainstate argument,
which in turn means it can be dropped from CreateAndProcessBlock. Using
the Mining interface here also requires marking the test
KernelNotifications chainstate as loaded after LoadVerifyActivateChainstate().

src/test/fuzz/process_message.cpp

@@ -42,7 +42,7 @@ void ResetChainman(TestingSetup& setup)
     setup.m_make_chainman();
     setup.LoadVerifyActivateChainstate();
     for (int i = 0; i < 2 * COINBASE_MATURITY; i++) {
-        node::BlockAssembler::Options options;
+        node::BlockCreateOptions options;
         MineBlock(setup.m_node, options);
     }
 }