stop provider when reseting providers

lib/tests/test.js

@@ -44,6 +44,9 @@ class Test {
   }
 
   initWeb3Provider(callback) {
+    if (this.provider) {
+      this.provider.stop();
+    }
     if (this.simOptions.host) {
       let {host, port, type, protocol, accounts}  = this.simOptions;
       if (!protocol) {
@@ -78,7 +81,9 @@ class Test {
         return {balance: account.hexBalance, secretKey: account.privateKey};
       });
     }
-    this.sim = getSimulator();
+    if (!this.sim) {
+      this.sim = getSimulator();
+    }
     this.web3.setProvider(this.sim.provider(this.simOptions));
     callback();
   }