CTestCustom: Suppress sphinx warning about missing favicon

There seems to be no way to avoid this warning on the first build
because Sphinx looks for the file before copying it into place.
I have not investigated deeply but it is simplest to just suppress
the warning from reports to CDash.
Merge topic 'CTestCustom-suppress-sphinx-icon-warning' into next

7f70ad89 CTestCustom: Suppress sphinx warning about missing favicon