refactor(transform_conformance): remove `TestCase` trait (#7356) #22508

Job	Run time
Build Linter Benchmark	2m 59s
Benchmark (lexer)	1m 20s
Benchmark (parser)	1m 48s
Benchmark (transformer)	2m 19s
Benchmark (isolated_declarations)	1m 46s
Benchmark (semantic)	1m 48s
Benchmark (minifier)	2m 17s
Benchmark (codegen)	1m 46s
Benchmark (sourcemap)	1m 34s
Benchmark linter (0)	3m 3s
Benchmark linter (1)	1m 34s
Upload benchmarks	24s
	22m 38s

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