refactor(transform_conformance): remove `TestCase` trait #22506

Job	Run time
Benchmark (lexer)	1m 35s
Build Linter Benchmark	2m 58s
Benchmark (parser)	1m 40s
Benchmark (transformer)	2m 26s
Benchmark (isolated_declarations)	1m 42s
Benchmark (semantic)	1m 58s
Benchmark (minifier)	2m 16s
Benchmark (codegen)	1m 38s
Benchmark (sourcemap)	1m 48s
Benchmark linter (0)	3m 1s
Benchmark linter (1)	1m 39s
Upload benchmarks	1s
	22m 42s

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