libstdc++-v3: Run prettyprinters.exp

[Paolo Carlini]

Hi,

> prettyprinters.exp is never run except in non-parallel check or when
> requested explicitly.  Tested on powerpc-linux and checked in as
> obvious.

Thanks Andreas.

Since you seem to pay good attention to these testing details, I wonder if you could also help with the issue I noticed again over the last days with make check-performance not actually doing much if make check isn't run before. I bet a fix would turn out to be just a few lines..

Paolo