When shelling out for a CLI test the error returned from the Start()
command prints the exit code which is not very informative. Capturing
and printing stderr is a lot more useful.
These had the generic name osbulid-tests, rename the binary to more
closely describe what is under test: weldr API as composed by
composer-cli.
Signed-off-by: Tom Gundersen <teg@jklm.no>