Re: psql - improve test coverage from 41% to 88%

Attached is a rebase after TestLib.pm got a documentation in 6fcc40b1.

> The attached patch improves psql code coverage by adding a specific TAP test. 
> The 1709 tests take 4 seconds CPU (6.3 elapsed time) on my laptop.
>
> The infrastructure is updated to require perl module "Expect", allowing to 
> test interactive features such as tab completion and prompt changes.
>
> Coverage in "src/bin/psql" jumps from 40.0% of lines and 41.9% of functions 
> to 78.4% of lines and 98.1% of functions with "check-world".

-- 
Fabien.