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Check for cd9660 support before attempting to mount created images

Description

Check for cd9660 support before attempting to mount created images

This extends the set in r316028 to allow all tests to pass or be skipped
on a system without cd9660 support.

A better approach using tar is possible, but this works today.

Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D10516