Fix Jest open handle in DeltaBundler resolver test

Summary:
This test was leaving dangling open handles in Jest because the "resolver" (containing a whole instance of `DependencyGraph`, watcher and all) wasn't properly ended.

`resolver` is automatically cleaned up in `afterEach` with `resolver.end()`, but in this test we were successfully creating a resolver instance without assigning it to `resolver`, leaving it hanging.

Changelog: Internal

Reviewed By: huntie

Differential Revision: D48783493

fbshipit-source-id: 711a2bc5879f4eb093e8ddf2fbc71a2804c7d976

packages/metro/src/DeltaBundler/__tests__/resolver-test.js

@@ -2034,7 +2034,7 @@ function dep(name: string): TransformResultDependency {
             },
           });
 
-          await expect(createResolver(config)).resolves;
+          resolver = await createResolver(config);
           expect(console.error).not.toHaveBeenCalled();
         });
       });