fix(test): resolve a timing issue in mempool test

[arya2]

Motivation

To fix a mempool test that's failing occasionally.

The prior mempool.dummy_calls should also see the chain tip changes, and it's possible for the wait_for_tip_change call after the third block commit in this test to see the tip change from the first or second block commits.

See #5322, closes #5301

Solution

Await wait_for_tip_change after every block commit.

Review

Part of regular scheduled work.

Reviewer Checklist

• Will the PR name make sense to users?
  • Does it need extra CHANGELOG info? (new features, breaking changes, large changes)
• Are the PR labels correct?
• Does the code do what the ticket and PR says?
• How do you know it works? Does it have tests?

[codecov]

Codecov Report

Merging #5347 (8208ea6) into main (9f6a1fd) will increase coverage by 0.02%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5347      +/-   ##
==========================================
+ Coverage   79.09%   79.12%   +0.02%     
==========================================
  Files         308      308              
  Lines       39560    39560              
==========================================
+ Hits        31291    31302      +11     
+ Misses       8269     8258      -11     

[teor2345]

Thanks!