apollo-server-lambda tests: adapt mock ALB server for upgrade

CodeGenieApp/serverless-express#392 makes the handler
return headers in the field matching the one where it receives request
headers: `headers` or `multiValueHeaders`. (I guess this is a preference
for ALB?) Our mock server needs to be consistent in which one it looks
at. This is a test-only change.

packages/apollo-server-lambda/src/__tests__/mockALBServer.ts

@@ -38,14 +38,9 @@ export function createMockServer(handler: ALBHandler) {
         },
       )) as ALBResult;
       res.statusCode = result.statusCode!;
-      Object.entries(result.multiValueHeaders ?? {}).forEach(
-        ([key, values]) => {
-          res.setHeader(
-            key,
-            values.map((v) => v.toString()),
-          );
-        },
-      );
+      Object.entries(result.headers ?? {}).forEach(([key, value]) => {
+        res.setHeader(key, value.toString());
+      });
       res.write(result.body);
       res.end();
     });