While looking over the code I noticed that the `Restore()` helper
is not needed (and arguably wrong as it does not reset PATH).
We already use `t.TempDir()` and `t.Setenv()` as part of the
command setup so manually cleanup is not neccessary (and is today
even incomplete). So YAGNI and we can remove Restore().
This commit adds a new testutil.CaptureStdio helper so that we
can test external go modules that use os.Std{out,err} but now
allow mocking or overwriting.
This commit adds a new testutil.MockCommand() helper that will
mock a command in $PATH to allow easier testing of e.g. the
`image-builder-cli build` comamnd that will invoke osbuild.
