Fix spurious crasher in mempool fuzz test. (#7190)

We were creating a TestLogger without testing being initialized.
Switch to a no-op logger instead.

test/fuzz/mempool/checktx.go

@@ -28,7 +28,7 @@ func init() {
 	getMp = func() mempool.Mempool {
 		if mp == nil {
 			mp = mempool.NewTxMempool(
-				log.TestingLogger().With("module", "mempool"),
+				log.NewNopLogger(),
 				cfg,
 				appConnMem,
 				0,