stats: improve TestCreateStatsUsingExtremesProgress a bit

[yuzefovich]

This commit addresses two problems with TestCreateStatsUsingExtremesProgress:

• previously, it was possible for the main goroutine to get stuck when retrieving the stats job progress because the progress was nil. My guess is that this was due to us sending on the channel that we block scans on only twice total before attempting to retrieve the progress. In the store filter we receive twice from the channel, so it seems like we could have a race between the first scanned row being processed and progress being written in one goroutine against running out of SucceedsSoon duration of 45s in another. We now send on the channel ten times before retrieving the progress for the first time which should prevent this race. (This is similar to what we do in TestCreateStatsProgress. I also tried sending only three times, but the problem was still present.)
• if we hit the succeeds soon error as described above, we would previously incorrectly shut down the test. This was the case because the test server would be stuck in the store filter we installed, and our attempt at closing the necessary channel was deferred before we defer the call to stop.Stopper.Stop which blocks until all tasks exit, but the server couldn't exit because it had one goroutine stuck on the channel. We fix this by deferring the closure of the channel after deferring the Stop call. The same problem was also present in TestCreateStatsProgress which is also now fixed.

Fixes: #135733.

Release note: None

[cockroach-teamcity]

This change is 

[yuzefovich]

TFTR!

bors r+

[craig[bot]]

Build succeeded:

• local_roachtest_fips
• macos_arm64_build
• check_generated_code
• acceptance
• linux_amd64_fips_build
• linux_arm64_build
• lint
• examples_orms
• windows_build
• linux_amd64_build
• docker_image_amd64
• macos_amd64_build
• unit_tests
• local_roachtest

[blathers-crl[bot]]

Based on the specified backports for this PR, I applied new labels to the following linked issue(s). Please adjust the labels as needed to match the branches actually affected by the issue(s), including adding any known older branches.

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Issue #135733: branch-release-24.3.

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