Add tests for mempool persistence

Adds tests for mempool persistence as well as for the new
-persistmempool command line parameter.

test/functional/test_runner.py

@@ -59,6 +59,7 @@ BASE_SCRIPTS= [
     'rest.py',
     'mempool_spendcoinbase.py',
     'mempool_reorg.py',
+    'mempool_persist.py',
     'httpbasics.py',
     'multi_rpc.py',
     'proxy_test.py',