fix: Update false-negative test case in Authentication.test

[adebayor123]

Details

Addresses the concern from #436.

nit: This test will pass under certain conditions when it should fail (i.e., false negative).

Specifically, when this.credentials.expiration is undefined, and the expiration property of the value returned by AnonymousStorageCredentialsProvider is a Date object. When this happens, control flow will reach AnonymousCognitoCredentialsProvider, and the test needs a way to distinguish the value returned here from the expected value.

You can fix this by mocking the response of getCredentials, like was done in EnhancedAuthentication.test.ts here.

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