pgsql: Make detach-partition-concurrently-4 less timing sensitive

Make detach-partition-concurrently-4 less timing sensitive

Same as 5e0b1aeb2dfe, for the companion test file.

This one seems lower probability (only two failures in a month of runs);
I was hardly able to reproduce a failure without a patch, so the fact
that I was also unable to reproduce one with it doesn't say anything.
We'll have to wait for further buildfarm results to see if we need any
further adjustments.

Discussion: https://postgr.es/m/20210524090712.GA3771394@rfd.leadboat.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/eb43bdbf5104c183412aac0fccf8e515e60d9212

Modified Files
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.../expected/detach-partition-concurrently-4.out | 90 ++++++++++++----------
.../specs/detach-partition-concurrently-4.spec | 29 ++++---
2 files changed, 68 insertions(+), 51 deletions(-)