Replace deprecated std::shared_ptr::unique

Summary:
Replace deprecated [std::shared_ptr<T>::unique](https://en.cppreference.com/w/cpp/memory/shared_ptr/unique) with `::use_count`

Internal
Fixes macos build

Reviewed By: vitaut

Differential Revision: D69187609

fbshipit-source-id: 42108414d508623046957494b316711b1b2a69a0

thrift/lib/cpp2/test/util/ScopedServerThreadTest.cpp

@@ -56,5 +56,5 @@ TEST(ScopedServerThreadTest, BindFailure) {
   server->setAddress(address);
   EXPECT_THROW(std::make_unique<ScopedServerThread>(server), exception);
   // Make sure there wasn't a leak of the ThriftServer, (cf. t13139338).
-  EXPECT_TRUE(server.unique());
+  EXPECT_EQ(1, server.use_count());
 }