add a limit and a test to FlattenExceptionTest.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #13003
| License       | MIT
| Doc PR        |
Fixed ... still can't get used to PSR-2