Doc: update testing recipe in src/test/perl/README.

Doc: update testing recipe in src/test/perl/README.

The previous text didn't provide any clear explanation of our policy
around TAP test portability.  The recipe for using perlbrew had some
problems, too: it resulted in a non-shared libperl (preventing
testing of plperl) and it caused some modules to be updated to
current when the point of the recipe is to build an old environment.

Discussion: https://postgr.es/m/E1mYY6Z-0006OL-QN@gemulon.postgresql.org