fix: flaky test_copy_dash test in dashboard_tests.py

[villebro]

SUMMARY

Fix a flaky unit test that was caused by the changed_on_humanized property in AuditMixinNullable: https://github.com/apache/incubator-superset/blob/315518d2d22a63de1b2e5f556e7fb5737669ebd6/superset/models/helpers.py#L383-L385

BEFORE

TEST PLAN

CI

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[codecov-commenter]

Codecov Report

Merging #10021 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10021      +/-   ##
==========================================
- Coverage   70.41%   70.37%   -0.05%     
==========================================
  Files         585      585              
  Lines       31056    31056              
  Branches     3277     3277              
==========================================
- Hits        21869    21856      -13     
- Misses       9076     9089      +13     
  Partials      111      111              

Flag	Coverage Δ
#cypress	53.93% <ø> (ø)
#javascript	59.36% <ø> (ø)
#python	69.92% <ø> (-0.08%)	⬇️

Impacted Files	Coverage Δ
superset/db_engine_specs/mysql.py	78.26% <0.00%> (-13.05%)	⬇️
superset/models/core.py	85.42% <0.00%> (-0.88%)	⬇️
superset/views/core.py	76.06% <0.00%> (-0.22%)	⬇️
superset/connectors/sqla/models.py	88.80% <0.00%> (-0.15%)	⬇️

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