feat: remove legacy tests, improve serde based ones #782

Job	Run time
go (oldstable)	22s
rust (stable, Cargo.toml)	19s
go (stable)	24s
Node.js (18.x, macos-latest)	18s
rust (stable, Cargo_latest_git.toml)	26s
rust (nightly, Cargo.toml)	25s
Node.js (18.x, ubuntu-latest)	12s
Node.js (18.x, windows-latest)	53s
rust (nightly, Cargo_latest_git.toml)	25s
Node.js (20.x, macos-latest)	16s
Node.js (20.x, ubuntu-latest)	19s
Node.js (20.x, windows-latest)	41s
Node.js (lts/*, macos-latest)	19s
Node.js (lts/*, ubuntu-latest)	17s
Node.js (lts/*, windows-latest)	45s
Node.js (current, macos-latest)	34s
Node.js (current, ubuntu-latest)	14s
Node.js (current, windows-latest)	27s
	7m 36s

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