pgsql: Make PG_TEST_NOCLEAN work for temporary directories in TAP tests

Make PG_TEST_NOCLEAN work for temporary directories in TAP tests

When set, this environment variable was only effective for data
directories but not for all the other temporary files created by
PostgreSQL::Test::Utils. Keeping the temporary files after a successful
run can be useful for debugging purposes.

The documentation is updated to reflect the new behavior, with contents
available in doc/ since v16 and in src/test/perl/README since v15.

Author: Jacob Champion
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/CAAWbhmgHtDH1SGZ+Fw05CsXtE0mzTmjbuUxLB9mY9iPKgM6cUw@mail.gmail.com
Discussion: https://postgr.es/m/[email protected]
Backpatch-through: 11

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/af36c13a7e92863413c433d3acd8aea62b216e9a

Modified Files
--------------
src/test/perl/TestLib.pm | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)