Avoid gcc7 bug in SerialExecutor test

Summary:
[Folly] Avoid gcc7 lambda-captures bug in `SerialExecutor` test.

```
folly/executors/test/SerialExecutorTest.cpp: In lambda function:
folly/executors/test/SerialExecutorTest.cpp:66:40: error: declaration of 'i' shadows a previous local [-Werror=shadow]
folly/executors/test/SerialExecutorTest.cpp:60:12: note: shadowed declaration is here
folly/executors/test/SerialExecutorTest.cpp: In function 'void simpleTest(const std::shared_ptr<folly::Executor>&)':
folly/executors/test/SerialExecutorTest.cpp:74:45: error: already captured 'i' in lambda expression [-Werror]
folly/executors/test/SerialExecutorTest.cpp: In lambda function:
folly/executors/test/SerialExecutorTest.cpp:74:62: error: 'i' is not captured
folly/executors/test/SerialExecutorTest.cpp:74:46: note: the lambda has no capture-default
folly/executors/test/SerialExecutorTest.cpp:60:12: note: 'int i' declared here
```

Fixes #1154.

Reviewed By: andriigrynenko

Differential Revision: D15629657

fbshipit-source-id: fec82e9d54b2c79a7f54f366ec8a19ad7dc198e9

folly/executors/test/SerialExecutorTest.cpp

@@ -63,7 +63,7 @@ void simpleTest(std::shared_ptr<folly::Executor> const& parent) {
         SerialExecutorContextData::kCtxKey(),
         std::make_unique<SerialExecutorContextData>(i));
     folly::RequestContextScopeGuard ctxGuard(ctx);
-    static auto checkReqCtx = [](auto i) {
+    auto checkReqCtx = [i] {
       EXPECT_EQ(
           i,
           dynamic_cast<SerialExecutorContextData*>(
@@ -71,8 +71,8 @@ void simpleTest(std::shared_ptr<folly::Executor> const& parent) {
                   SerialExecutorContextData::kCtxKey()))
               ->getId());
     };
-    executor->add([i, g = folly::makeGuard([i] { checkReqCtx(i); }), &values] {
-      checkReqCtx(i);
+    executor->add([i, checkReqCtx, g = folly::makeGuard(checkReqCtx), &values] {
+      checkReqCtx();
       // make this extra vulnerable to concurrent execution
       values.push_back(0);
       burnMs(10);