Make omitted --old-iso default to --iso in the automated test suite
Instead of erroring out, let’s just make the old Tails ISO be the same as the ISO being tested when —old-iso is omitted. This will sill results in us testing the most important thing we want to test, i.e. that the Tails Installer works in the ISO being tested.
Then we can also remove the @
Feature Branch: test/10147-default-old-iso