psql - improve test coverage from 41% to 88%

Hello devs,

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.