Refactor Cambodia and Laos test cases

[PPsyrius]

Proposed change

As proposed by @KJhellico in #2081 :

Now that 2058 is no longer a special year for Thai calendar, can we replace it with a more standard 2050? Or rather, 2049, because most countries use range(N, 2050).

I think the checks for 2158 are more related to Thai calendar tests than to Thailand itself. And since tests\calendars\test_thai.py already contains checks for this year, they are redundant here.

This PR aims to address these issues.

• Switching test case end date back to 2050 like other typical country-level implementations.
• Moving all relevant Thai calendar-related testing to Thai calendar test cases, except for Pchum Ben and Bon Om Tok since we'll still need to check for its 2-day and 3-day celebration variants.

Type of change

• New country/market holidays support (thank you!)
• Supported country/market holidays update (calendar discrepancy fix, localization)
• Existing code/documentation/test/process quality improvement (best practice, cleanup, refactoring, optimization)
• Dependency update (version deprecation/pin/upgrade)
• Bugfix (non-breaking change which fixes an issue)
• Breaking change (a code change causing existing functionality to break)
• New feature (new holidays functionality in general)

Checklist

• I've followed the contributing guidelines
• I've run make pre-commit, it didn't generate any changes
• I've run make test, all tests passed locally

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (b2e6837) to head (35597d9).
Report is 2 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff            @@
##               dev     #2086   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          191       191           
  Lines        11547     11545    -2     
  Branches      1744      1742    -2     
=========================================
- Hits         11547     11545    -2     

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[KJhellico]

LGTM.

[arkid15r]

👍