doc: Document variables for path substitution in SQL tests

doc: Document variables for path substitution in SQL tests

Test suites driven by pg_regress can use the following environment
variables for path substitutions since d1029bb5a26c:
- PG_ABS_SRCDIR
- PG_ABS_BUILDDIR
- PG_DLSUFFIX
- PG_LIBDIR

These variables have never been documented, and they can be useful for
out-of-core code based on the options used by the pg_regress command
invoked by installcheck (or equivalent) to build paths to libraries for
various commands, like LOAD or CREATE FUNCTION.

Reviewed-by: Zhang Hu <kongbaik228@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/abDAWzHaesHLDFke@paquier.xyz