Coinbases-in-mempool regression test

Immature coinbase spends are allowed in the memory pool if they can be mined in the next block.
They are not allowed in the memory pool if they cannot be mined in the next block.

This regression test tests those edge cases.

qa/pull-tester/rpc-tests.sh

@@ -22,6 +22,7 @@ if [ "x${ENABLE_BITCOIND}${ENABLE_UTILS}${ENABLE_WALLET}" = "x111" ]; then
   ${BUILDDIR}/qa/rpc-tests/txn_doublespend.py --mineblock --srcdir "${BUILDDIR}/src"
   ${BUILDDIR}/qa/rpc-tests/getchaintips.py --srcdir "${BUILDDIR}/src"
   ${BUILDDIR}/qa/rpc-tests/rest.py --srcdir "${BUILDDIR}/src"
+  ${BUILDDIR}/qa/rpc-tests/mempool_spendcoinbase.py --srcdir "${BUILDDIR}/src"
   #${BUILDDIR}/qa/rpc-tests/forknotify.py --srcdir "${BUILDDIR}/src"
 else
   echo "No rpc tests to run. Wallet, utils, and bitcoind must all be enabled"