mirror: add reaction to snapshot test query (#1615)

Summary:
Upcoming changes will add support for field-level fidelity annotations
(see #998), at which point the `Reaction.user` field will be marked
unfaithful. This patch will surface that behavior change.

Test Plan:
The newly snapshotted query is valid, and returns a reaction whose
`user` property lists typename and ID.

wchargin-branch: mirror-snapshot-reaction

src/graphql/__snapshots__/mirror.test.js.snap

@@ -259,6 +259,17 @@ exports[`graphql/mirror Mirror _updateOwnData snapshot test for actual GitHub qu
       title
     }
   }
+  reaction: node(id: \\"MDg6UmVhY3Rpb24yOTUzNDM4Mw==\\") {
+    ... on Reaction {
+      __typename
+      id
+      content
+      user {
+        __typename
+        id
+      }
+    }
+  }
   commit: node(id: \\"MDY6Q29tbWl0MTIwMTQ1NTcwOjU1OTUwZjUzNTQ1NTEwOWJhNDhhYmYyYjk3N2U2NmFhMWNjMzVlNjk=\\") {
     ... on Commit {
       __typename

src/graphql/mirror.test.js

@@ -2364,6 +2364,7 @@ describe("graphql/mirror", () => {
         const db = new Database(":memory:");
         const mirror = new Mirror(db, buildGithubSchema());
         const exampleIssueId = "MDU6SXNzdWUzNDg1NDA0NjE=";
+        const exampleReactionId = "MDg6UmVhY3Rpb24yOTUzNDM4Mw==";
         const exampleCommitId =
           "MDY6Q29tbWl0MTIwMTQ1NTcwOjU1OTUwZjUzNTQ1NTEwOWJhNDhhYmYyYjk3N2U2NmFhMWNjMzVlNjk=";
         const b = Queries.build;
@@ -2377,6 +2378,12 @@ describe("graphql/mirror", () => {
                 b.inlineFragment("Issue", mirror._queryOwnData("Issue")),
               ])
             ),
+            b.alias(
+              "reaction",
+              b.field("node", {id: b.literal(exampleReactionId)}, [
+                b.inlineFragment("Reaction", mirror._queryOwnData("Reaction")),
+              ])
+            ),
             b.alias(
               "commit",
               b.field("node", {id: b.literal(exampleCommitId)}, [