3291 follow-up (#3323)

* changelog: use issue number instead of PR number

* follow up to #3291

- rpc/test/helpers.go add StopTendermint(node) func
- remove ensureDir(filepath.Dir(walFile), 0700)
- mempool/mempool_test.go add type cleanupFunc func()

* cmd/show_validator: wrap err to make it more clear

mempool/mempool_test.go

@@ -25,7 +25,11 @@ import (
 	"github.com/tendermint/tendermint/types"
 )
 
-func newMempoolWithApp(cc proxy.ClientCreator) (*Mempool, func()) {
+// A cleanupFunc cleans up any config / test files created for a particular
+// test.
+type cleanupFunc func()
+
+func newMempoolWithApp(cc proxy.ClientCreator) (*Mempool, cleanupFunc) {
 	config := cfg.ResetTestRoot("mempool_test")
 
 	appConnMem, _ := cc.NewABCIClient()