Use new connection for every test case

src/objective-c/tests/InteropTests/InteropTests.m

@@ -364,6 +364,11 @@ initWithInterceptorManager:(GRPCInterceptorManager *)interceptorManager
 
   [GRPCCall resetHostSettings];
 
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
+  [GRPCCall closeOpenConnections];
+#pragma clang diagnostic pop
+
   _service = [[self class] host] ? [RMTTestService serviceWithHost:[[self class] host]] : nil;
 }
 

src/objective-c/tests/MacTests/StressTests.m

@@ -118,6 +118,11 @@ extern const char *kCFStreamVarName;
 
   [GRPCCall resetHostSettings];
 
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
+  [GRPCCall closeOpenConnections];
+#pragma clang diagnostic pop
+
   GRPCMutableCallOptions *options = [[GRPCMutableCallOptions alloc] init];
   options.transportType = [[self class] transportType];
   options.PEMRootCertificates = [[self class] PEMRootCertificates];