TestIso: rename create_test_iso() to create_fake_iso()
TestIso: rename create_test_iso() to create_fake_iso()

This function creates a simple file with a name that ends in ".iso", but
since we are avoiding the use of `genisoimage` here, it is not a real
"ISO" file at all.

Rename the function to indicate this.

alfredodeza/merfi