Fix Panel_IFrame - FTP URL not allowed in 0.63

[ReneNulschDE]

Description:

FTP links are not allowed in 0.63. Changed code back to vol.url() test.
Additional test added.

Related issue (if applicable): fixes #12286

Example entry for configuration.yaml (if applicable):

panel_iframe:
  router:
    title: 'Router'
    url: 'ftp://192.168.1.1'

Checklist:

• The code change is tested and works locally.

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works.