Show backtrace when tap tests fail

Hi,

I've had a couple cases where tap tests died, and I couldn't easily see
where / why. For development of a new test I found it useful to show
backtraces in that case - just adding a
use Carp::Always;
at the start of the relevant module did the trick.

I'm wondering if we shouldn't always do so if the module is
installed. I.e. have PostgresNode or something do something like

# Include module showing backtraces upon failures. As it's a
non-standard module, don't fail if not installed.
eval { use Carp::Always; }

Comments?

- Andres