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

src/test/perl/README

@@ -61,9 +61,17 @@ Test::More::like entails use of the qr// operator.  Avoid Perl 5.8.8 bug
 #39185 by not using the "$" regular expression metacharacter in qr// when also
 using the "/m" modifier.  Instead of "$", use "\n" or "(?=\n|\z)".
 
-Read the Test::More documentation for more on how to write tests:
+Test::Builder::Level controls how far up in the call stack a test will look
+at when reporting a failure.  This should be incremented by any subroutine
+which directly or indirectly calls test routines from Test::More, such as
+ok() or is():
+
+    local $Test::Builder::Level = $Test::Builder::Level + 1;
+
+Read the documentation for more on how to write tests:
 
     perldoc Test::More
+    perldoc Test::Builder
 
 For available PostgreSQL-specific test methods and some example tests read the
 perldoc for the test modules, e.g.: