test: freeze time for dashboard export test

[ktmud]

SUMMARY

This fix a flaky test fail where server side time vs test case assertion time did not happen within 1 second:

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #19634 (b6126a0) into master (9a9e3b6) will decrease coverage by 0.16%.
The diff coverage is n/a.

❗ Current head b6126a0 differs from pull request most recent head 0db5120. Consider uploading reports for the commit 0db5120 to get more accurate results

@@            Coverage Diff             @@
##           master   #19634      +/-   ##
==========================================
- Coverage   66.49%   66.32%   -0.17%     
==========================================
  Files        1681     1681              
  Lines       64370    64370              
  Branches     6583     6583              
==========================================
- Hits        42803    42695     -108     
- Misses      19886    19994     +108     
  Partials     1681     1681              

Flag	Coverage Δ
hive	?
mysql	81.89% <ø> (ø)
postgres	81.94% <ø> (ø)
presto	?
python	82.03% <ø> (-0.35%)	⬇️
sqlite	81.71% <ø> (ø)
unit	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/db_engines/hive.py	0.00% <0.00%> (-85.19%)	⬇️
superset/db_engine_specs/hive.py	70.11% <0.00%> (-15.71%)	⬇️
superset/db_engine_specs/presto.py	83.64% <0.00%> (-5.39%)	⬇️
superset/connectors/sqla/models.py	88.24% <0.00%> (-1.22%)	⬇️
superset/initialization/__init__.py	91.31% <0.00%> (-0.35%)	⬇️
superset/db_engine_specs/base.py	88.01% <0.00%> (-0.34%)	⬇️
superset/models/core.py	88.83% <0.00%> (-0.24%)	⬇️
superset/utils/core.py	89.71% <0.00%> (-0.12%)	⬇️

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[zhaoyongjie]

LGTM