pgsql: doc: document the TAP test environment variables

doc: document the TAP test environment variables

The TAP tests can, to some degree, be controlled by a set of environment
variables. These were however only documented in a README and not in the
main documentation. This adds documentation of these variables, as well
as changes one CPAN reference to a ulink for consistency. While there,
also tag CPAN as an acronym as it's listed in the acronyms section.

Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/YyPd9unV14SX2bLF@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c4421550fb57ee12369938197aeb1cd189142453

Modified Files
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doc/src/sgml/install-windows.sgml | 8 +++++++-
doc/src/sgml/regress.sgml | 28 +++++++++++++++++++++++++++-
2 files changed, 34 insertions(+), 2 deletions(-)