Incrementing the level of the call stack reported is useful for
debugging purposes as it allows to control which part of the test is
exactly failing, especially if a test is structured with subroutines
that call routines from Test::More.

This adds more incrementations of $Test::Builder::Level where debugging
gets improved (for example it does not make sense for some paths like
pg_rewind where long subroutines are used).

A note is added to src/test/perl/README about that, based on a
suggestion from Andrew Dunstan and a wording coming from both of us.

Usage of Test::Builder::Level has spread in 12, so a backpatch down to
this version is done.

Reviewed-by: Andrew Dunstan, Peter Eisentraut, Daniel Gustafsson
Discussion: https://postgr.es/m/YV1CCFwgM1RV1LeS@paquier.xyz
Backpatch-through: 12