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