Add correct test for warnings logged in restricted iframes

[josegonzalez]

Describe your context
Please provide us your environment so we can easily reproduce the issue.

• replace the result of pip list | grep dash below

dash                 1.8.0
dash-core-components 1.7.1
dash-html-components 1.0.2
dash-renderer        1.2.3
dash-table           4.6.0

• if frontend related, tell us your Browser, Version and OS

  • OS: All
  • Browser: All

Describe the bug

With the change in #1080 to allow usage within restricted iframes, we also do a somewhat poor job of checking the type of the warnings logged.

Expected behavior

We should check the exact error string if possible, or at least check for the existence of a specific substring.