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@paquier.xyz>
Discussion: https://postgr.es/m/YyPd9unV14SX2bLF@paquier.xyz