Add more $Test::Builder::Level in the TAP tests

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

contrib/test_decoding/t/001_repl_stats.pl

@@ -19,6 +19,8 @@ $node->start;
 # Check that replication slot stats are expected.
 sub test_slot_stats
 {
+	local $Test::Builder::Level = $Test::Builder::Level + 1;
+
 	my ($node, $expected, $msg) = @_;
 
 	my $result = $node->safe_psql(