address TypeScript compilation failure

This fixes...

```
Run yarn tsc
yarn run v1.22.1
$ tsc
plugins/splunk-on-call/src/components/EntitySplunkOnCallCard.test.tsx:158:26 - error TS2345: Argument of type 'string | undefined' is not assignable to parameter of type 'Matcher'.
  Type 'undefined' is not assignable to type 'Matcher'.

158         expect(getByText(MOCK_INCIDENT.entityDisplayName)).toBeInTheDocument(),
                             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Signed-off-by: Mike Ball <mikedball@gmail.com>

plugins/splunk-on-call/src/components/EntitySplunkOnCallCard.test.tsx

@@ -154,8 +154,7 @@ describe('SplunkOnCallCard', () => {
     await waitFor(() => !queryByTestId('progress'));
     expect(getByText(`Team: ${MOCK_TEAM.name}`)).toBeInTheDocument();
     await waitFor(
-      () =>
-        expect(getByText(MOCK_INCIDENT.entityDisplayName)).toBeInTheDocument(),
+      () => expect(getByText('test-incident')).toBeInTheDocument(),
       { timeout: 2000 },
     );
   });