Add LegacyApp and Benchmark API

[Kyle-Ye]

No description provided.

[codecov-commenter]

Codecov Report

Attention: 95 lines in your changes are missing coverage. Please review.

Comparison is base (75efab0) 16.98% compared to head (70721ef) 16.32%.

❗ Current head 70721ef differs from pull request most recent head 6b04c2d. Consider uploading reports for the commit 6b04c2d to get more accurate results

Files	Patch %	Lines
Sources/OpenSwiftUI/Test/_BenchmarkHost.swift	0.00%	31 Missing ⚠️
Sources/OpenSwiftUI/Test/_PerformanceTest.swift	0.00%	22 Missing ⚠️
Sources/OpenSwiftUI/Test/TestApp.swift	0.00%	12 Missing ⚠️
...iftUI/AppStructure/AppOrganization/LegacyApp.swift	0.00%	8 Missing ⚠️
Sources/OpenSwiftUI/Test/TestIDView.swift	0.00%	8 Missing ⚠️
...OpenSwiftUI/Internal/Other/EnvironmentHelper.swift	0.00%	5 Missing ⚠️
...SwiftUI/Views/View/internal/ViewRendererHost.swift	0.00%	4 Missing ⚠️
Sources/OpenSwiftUI/Test/_Benchmark.swift	0.00%	3 Missing ⚠️
Sources/OpenSwiftUI/Test/_Test.swift	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #25      +/-   ##
==========================================
- Coverage   16.98%   16.32%   -0.66%     
==========================================
  Files         139      148       +9     
  Lines        2278     2370      +92     
==========================================
  Hits          387      387              
- Misses       1891     1983      +92     

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