folly/futures/test/ViaTest.cpp: avoid shadowing warnings

Summary: Fix warnings exposed by the upstream-proposed -Wshadow-compatible-local option.

Reviewed By: igorsugak

Differential Revision: D4041769

fbshipit-source-id: 257bfe9370279f755b8e965ffc1c37fef454d4c4

folly/futures/test/ViaTest.cpp

@@ -143,9 +143,9 @@ TEST_F(ViaFixture, chainVias) {
     return 1;
   }).then([=](int val) {
     return makeFuture(val).via(westExecutor.get())
-      .then([=](int val) mutable {
+      .then([=](int v) mutable {
         EXPECT_EQ(std::this_thread::get_id(), westThreadId);
-        return val + 1;
+        return v + 1;
       });
   }).then([=](int val) {
     // even though ultimately the future that triggers this one executed in