Disable failing files app test

Fails with:

 * tests/acceptance/features/app-files.feature:90
<details><summary>Show full log</summary>

```
  Scenario: show favorites                                            # /drone/src/github.com/nextcloud/server/tests/acceptance/features/app-files.feature:90
    Given I am logged in                                              # LoginPageContext::iAmLoggedIn()
    And I mark "welcome.txt" as favorite                              # FileListContext::iMarkAsFavorite()
    When I open the "Favorites" section                               # AppNavigationContext::iOpenTheSection()
    Then I see that the current section is "Favorites"                # AppNavigationContext::iSeeThatTheCurrentSectionIs()
    Then I see that the file list contains a file named "welcome.txt" # FileListContext::iSeeThatTheFileListContainsAFileNamed()
      Row for file welcome.txt in file list could not be found after 100 seconds (NoSuchElementException)
```
</details>

Signed-off-by: Morris Jobke <hey@morrisjobke.de>