pgsql: Add test for dead-end backends

Add test for dead-end backends

The code path for launching a dead-end backend because we're out of
slots was not covered by any tests, so add one. (Some tests did hit
the case of launching a dead-end backend because the server is still
starting up, though, so the gap in our test coverage wasn't as big as
it sounds.)

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/a102f15f-eac4-4ff2-af02-f9ff209ec66f@iki.fi

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/85ec945b7880931cb506392101cb0b00209b78ba

Modified Files
--------------
src/test/perl/PostgreSQL/Test/Cluster.pm       | 78 ++++++++++++++++++++++++++
src/test/postmaster/t/001_connection_limits.pl | 42 +++++++++++++-
2 files changed, 119 insertions(+), 1 deletion(-)