Move makeAsyncTask test helpers inline into the test

Summary: [Folly] Move `makeAsyncTask` test helpers inline into the test.

Reviewed By: ericniebler

Differential Revision: D22674219

fbshipit-source-id: 630c84ce153ad2bfb92d38f3d2c955cb3f043799

folly/futures/test/FutureTest.cpp

@@ -1330,12 +1330,14 @@ TEST(Future, makePromiseContract) {
   EXPECT_EQ(4, std::move(c.second).get());
 }
 
-Future<bool> call(int depth, Executor* executor) {
+TEST(Future, ThenRecursion) {
+  struct Helpers {
+    static Future<bool> call(int depth, Executor* executor) {
       return makeFuture().thenValueInline(
           makeAsyncTask(executor, [=](auto&&) { return depth == 0; }));
-}
+    }
 
-Future<int> recursion(Executor* executor, int depth) {
+    static Future<int> recursion(Executor* executor, int depth) {
       return makeFuture().thenValue([=](auto) {
         return call(depth, executor).thenValue([=](auto result) {
           if (result) {
@@ -1345,12 +1347,12 @@ Future<int> recursion(Executor* executor, int depth) {
           return recursion(executor, depth - 1);
         });
       });
-}
+    }
+  };
 
-TEST(Future, ThenRecursion) {
   ManualExecutor executor;
 
-  EXPECT_EQ(42, recursion(&executor, 100000).getVia(&executor));
+  EXPECT_EQ(42, Helpers::recursion(&executor, 100000).getVia(&executor));
 }
 
 // We want to detect if the Try value is being dereferenced before being