test runner should use --no-built-binaries

The rules that decide if a package needs to be built by adt-run are roughly:

<jibel> plars, if you run from a build tree it won't rebuild the package unless build-needed is specified
<plars> jibel: is there any way to tell it not to rebuild? For example, in the case of the kernel package?
<jibel> plars, IIRC --no-built-binaries does it
<plars> jibel: hmm, but does build-needed override that? It would seem harsh to set that by default since some packages, I guess, do expect that they are going to need to be built?
<jibel> plars, it does

The test runner invokes adt-run after getting the sources (from the archive, we may want to get them from the ticket itself but for now the branch source builder uploading them to lp and the image builder setting the right ppas give the same end result).

So this needs testing but would avoid rebuilding the kernel as it does right now.