Fix folly/test/small_vector_test.cpp under gcc8 with -Wshadow

Summary:
[Folly] Fix `folly/test/small_vector_test.cpp` under gcc8 with `-Wshadow`.

```
folly/test/small_vector_test.cpp: In member function 'virtual void small_vector_NoCopyCtor_Test::TestBody()':
folly/test/small_vector_test.cpp:898:10: error: declaration of 'struct small_vector_NoCopyCtor_Test::TestBody()::Test' shadows a previous local [-Werror=shadow]
In file included from folly/portability/GTest.h:33,
                 from folly/test/small_vector_test.cpp:33:
third-party-buck/platform007/build/googletest/include/gtest/gtest.h:371:52: note: shadowed declaration is here
```

Reviewed By: Orvid

Differential Revision: D13603337

fbshipit-source-id: 84747335787df4c9288d93f19bd0629c53a68e4d

folly/test/small_vector_test.cpp

@@ -895,15 +895,15 @@ TEST(small_vector, InputIterator) {
 }
 
 TEST(small_vector, NoCopyCtor) {
-  struct Test {
-    Test() = default;
-    Test(const Test&) = delete;
-    Test(Test&&) = default;
+  struct Tester {
+    Tester() = default;
+    Tester(const Tester&) = delete;
+    Tester(Tester&&) = default;
 
     int field = 42;
   };
 
-  small_vector<Test> test(10);
+  small_vector<Tester> test(10);
   ASSERT_EQ(test.size(), 10);
   for (const auto& element : test) {
     EXPECT_EQ(element.field, 42);