fix(reports): Update the element class to wait for when taking a screenshot

[Vitor-Avila]

SUMMARY

In the process of taking screenshots for an alert/report, Selenium/Playwright is configured to wait a certain time (controlled via superset_config.py) until all chart elements are drawn. The logic was configured to monitor the slice_container class, however with #27255 this class is only used once the chart data is retrieved.

This PR changes the class to be monitored to the chart-container wrapper.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

No UI changes.

TESTING INSTRUCTIONS

1. Configure SCREENSHOT_LOCATE_WAIT to be 30 seconds, and SCREENSHOT_LOAD_WAIT to be 200 seconds.
2. Create a chart that takes longer than 30 seconds to load. You can use pg_sleep() with Postgres for example.
3. Configure a report with this chart.

Before this PR the report would fail, as the chart wouldn't load within 30 seconds.

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

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 83.47%. Comparing base (76d897e) to head (f8b39a9).
Report is 235 commits behind head on master.

Files	Patch %	Lines
superset/utils/webdriver.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #28745       +/-   ##
===========================================
+ Coverage   60.48%   83.47%   +22.98%     
===========================================
  Files        1931      523     -1408     
  Lines       76236    37575    -38661     
  Branches     8568        0     -8568     
===========================================
- Hits        46114    31365    -14749     
+ Misses      28017     6210    -21807     
+ Partials     2105        0     -2105     

Flag	Coverage Δ
hive	49.01% <0.00%> (-0.16%)	⬇️
javascript	?
mysql	77.12% <0.00%> (?)
postgres	77.23% <0.00%> (?)
presto	53.56% <0.00%> (-0.25%)	⬇️
python	83.47% <0.00%> (+19.98%)	⬆️
sqlite	76.68% <0.00%> (?)
unit	58.94% <0.00%> (+1.32%)	⬆️

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[kgabryje]

LGTM