tests: mock fetch call in pfe-icon

[bennypowers]

Description of the issue

tests involving pfe-icon are likely to actually fetch icon files from redhat production servers.

pfe-tools/test-runner should mock fetch to return local files for pfe-icons

Impacted component(s)

• anything using pfe-icon

Steps to reproduce

1. Run tests in CI
2. tests may fail with timeouts

Expected behavior

tests do not time out or make network calls