Error when testing v13 using apollo-server-express
I have uploading working, however I can't get a test working. When using `apollo-server`, the error is:
```
Variable "$file" got invalid value {}; Upload value invalid.
```
But when using `apollo-server-express`, the `query` must be a string, which isn't available to me after using `graphql-codegen`, and I can't use the `DocumentNode` which is generated.

Here's the test code:
```
    const file = Promise.resolve({
      createReadStream: () => stream,
      stream,
      filename,
      mimetype: 'text/plain',
      encoding: 'utf-8',
    })
await server.executeOperation({
        query: UploadDocumentFileDocument,
        variables: {
          file,
          documentId,
          fileId,
        },
      })
```

```
export const UploadDocumentFileDocument = ({
  kind: 'Document',
  definitions: [
    {
      kind: 'OperationDefinition',
      operation: 'mutation',
      name: { kind: 'Name', value: 'UploadDocumentFile' },
      variableDefinitions: [
        {
          kind: 'VariableDefinition',
          variable: { kind: 'Variable', name: { kind: 'Name', value: 'file' } },
          type: {
            kind: 'NonNullType',
            type: {
              kind: 'NamedType',
              name: { kind: 'Name', value: 'Upload' },
            },
          },
        },
        {
          kind: 'VariableDefinition',
          variable: {
            kind: 'Variable',
            name: { kind: 'Name', value: 'documentId' },
          },
          type: {
            kind: 'NonNullType',
            type: {
              kind: 'NamedType',
              name: { kind: 'Name', value: 'String' },
            },
          },
        },
        {
          kind: 'VariableDefinition',
          variable: {
            kind: 'Variable',
            name: { kind: 'Name', value: 'fileId' },
          },
          type: {
            kind: 'NonNullType',
            type: {
              kind: 'NamedType',
              name: { kind: 'Name', value: 'String' },
            },
          },
        },
      ],
      selectionSet: {
        kind: 'SelectionSet',
        selections: [
          {
            kind: 'Field',
            name: { kind: 'Name', value: 'uploadDocumentFile' },
            arguments: [
              {
                kind: 'Argument',
                name: { kind: 'Name', value: 'file' },
                value: {
                  kind: 'Variable',
                  name: { kind: 'Name', value: 'file' },
                },
              },
              {
                kind: 'Argument',
                name: { kind: 'Name', value: 'documentId' },
                value: {
                  kind: 'Variable',
                  name: { kind: 'Name', value: 'documentId' },
                },
              },
              {
                kind: 'Argument',
                name: { kind: 'Name', value: 'fileId' },
                value: {
                  kind: 'Variable',
                  name: { kind: 'Name', value: 'fileId' },
                },
              },
            ],
            selectionSet: {
              kind: 'SelectionSet',
              selections: [
                {
                  kind: 'FragmentSpread',
                  name: { kind: 'Name', value: 'DocumentFileFragment' },
                },
              ],
            },
          },
        ],
      },
    },
    {
      kind: 'FragmentDefinition',
      name: { kind: 'Name', value: 'DocumentFileFragment' },
      typeCondition: {
        kind: 'NamedType',
        name: { kind: 'Name', value: 'DocumentFile' },
      },
      selectionSet: {
        kind: 'SelectionSet',
        selections: [
          { kind: 'Field', name: { kind: 'Name', value: 'id' } },
          { kind: 'Field', name: { kind: 'Name', value: 'createdAt' } },
          { kind: 'Field', name: { kind: 'Name', value: 'updatedAt' } },
          { kind: 'Field', name: { kind: 'Name', value: 'documentId' } },
          { kind: 'Field', name: { kind: 'Name', value: 'fileId' } },
          { kind: 'Field', name: { kind: 'Name', value: 'filename' } },
          { kind: 'Field', name: { kind: 'Name', value: 'mimetype' } },
          { kind: 'Field', name: { kind: 'Name', value: 'encoding' } },
          { kind: 'Field', name: { kind: 'Name', value: 'url' } },
        ],
      },
    },
  ],
} as unknown) as DocumentNode
```