pgsql: Make sure to run pg_isready on correct port

Make sure to run pg_isready on correct port

The current code can have pg_isready unexpectedly succeed if there is a
server running on the default port. To avoid this we delay running the
test until after a node has been created but before it starts, and then
use that node's port, so we are fairly sure there is nothing running on
the port.

Backpatch to all live branches.

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/75f4ae2b8065ca7d29bec55f261828f794fbba1e

Modified Files
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src/bin/scripts/t/080_pg_isready.pl | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)