pgsql: Document PG_TEST_NOCLEAN in TAP test README

Document PG_TEST_NOCLEAN in TAP test README

Commit 90627cf98 added support for retaining the data directory even on
successful tests, but failed to document the environment variable which
controls retention. This adds a small note to the TAP test README about
PG_TEST_NOCLEAN which when set skips removing the data directories from
successful tests.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/2B02C1B3-3F41-4E14-92B9-005D83623A0B@yesql.se

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/05d8785af2a192d436df5b7734aacb4e0bab5da8

Modified Files
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src/test/perl/README | 5 +++++
1 file changed, 5 insertions(+)