Add allowed error message to integration testsuite's malformed JSON t…

…est (#7080)

I'm currently building a Next.js integration for Apollo Server 4 over
[here](https://github.com/apollo-server-integrations/apollo-server-integration-next).

To make sure the integration works properly with Next.js I'm reusing
Next.js's
[apiResolver](https://github.com/vercel/next.js/blob/canary/packages/next/server/api-utils/node.ts#L426),
which handles parsing API route requests. I [managed to make every
test](apollo-server-integrations/apollo-server-integration-next#14)
in `apollo-server-integrations-testsuite` pass except for one, `throws
an error if POST JSON is malformed`. This test checks the error message
in the server response when malformed JSON data is sent and matches it
to two possible substrings, `Unexpected token f` and `Bad Request`.

Next.js's `apiResolver` uses the
[parseBody](https://github.com/vercel/next.js/blob/canary/packages/next/server/api-utils/node.ts#L143)
function to parse the request body, which in turn uses
[parseJSON](https://github.com/vercel/next.js/blob/canary/packages/next/server/api-utils/node.ts#L126)
to parse JSON. In the case of malformed JSON `parseJSON` throws an
`ApiError` with the message `Invalid JSON`, which is then returned in
the server response. Since it's not possible for me to change the
message returned by Next.js I was hoping I could add it to the tests's
allowed error messages, making the test pass.

Co-authored-by: David Glasser <glasser@apollographql.com>

.changeset/many-teachers-remain.md

@@ -0,0 +1,5 @@
+---
+'@apollo/server-integration-testsuite': patch
+---
+
+Recognize malformed JSON error messages from Next.js.

packages/integration-testsuite/src/httpServerTests.ts

@@ -379,8 +379,8 @@ export function defineIntegrationTestSuiteHttpServerTests(
         return req.then((res) => {
           expect(res.status).toEqual(400);
           expect(
-            ['Unexpected token f', 'Bad Request'].some((substring) =>
-              (res.error as HTTPError).text.includes(substring),
+            ['Unexpected token f', 'Bad Request', 'Invalid JSON'].some(
+              (substring) => (res.error as HTTPError).text.includes(substring),
             ),
           ).toBe(true);
         });