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/YyPd9unV14SX2bLF@paquier.xyz
Backpatch-through: 11

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c24e9ef330ac7da1d1628d21b2d5defb0ec03c54

Modified Files
--------------
doc/src/sgml/regress.sgml              | 2 ++
src/test/perl/PostgreSQL/Test/Utils.pm | 7 ++++---
src/test/perl/README                   | 5 +++--
3 files changed, 9 insertions(+), 5 deletions(-)