[fix][test] Fix flaky NonPersistentTopicTest.testMsgDropStat

[pdolif]

Fixes #20386

Motivation

It seems that messages are dropped very fast, so the drop stats are already 0 again once we reach the assertion.

Modifications

• Sleep 10ms after producing a message. With this, the time it takes until all messages are produced is extended. The drop stats are > 0 for a longer time (and at the time when we reach the assertion).
• Increase the number of threads used for the producer from 5 to 10 to increase concurrency and the probability that messages are dropped.

Verifying this change

• Make sure that the change passes the CI checks.

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

Matching PR in forked repository

PR in forked repository: pdolif#9

[lhotari]

LGTM

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.26%. Comparing base (bbc6224) to head (436c26d).
Report is 998 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #24134      +/-   ##
============================================
+ Coverage     73.57%   74.26%   +0.68%     
+ Complexity    32624    32497     -127     
============================================
  Files          1877     1864      -13     
  Lines        139502   144453    +4951     
  Branches      15299    16479    +1180     
============================================
+ Hits         102638   107275    +4637     
+ Misses        28908    28724     -184     
- Partials       7956     8454     +498     

Flag	Coverage Δ
inttests	26.76% <ø> (+2.18%)	⬆️
systests	23.14% <ø> (-1.18%)	⬇️
unittests	73.77% <ø> (+0.93%)	⬆️

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see 1066 files with indirect coverage changes

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[lhotari]

Thanks for the contribution @pdolif, good work!