pgsql: 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: /messages/by-id/E1mYY6Z-0006OL-QN@gemulon.postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3eb1f4d09745433c70ccac411cad24d0374b9c3b

Modified Files
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src/test/perl/README | 32 ++++++++++++++++++++++++--------
1 file changed, 24 insertions(+), 8 deletions(-)