chore: remove `testnet` command #74

Job	Run time
build (windows-latest, x86_64-pc-windows-msvc)	2m 45s
Unused dependency check	47s
build (macos-latest, x86_64-apple-darwin)	1m 26s
Clippy & fmt	23s
build (ubuntu-latest, x86_64-unknown-linux-musl)	1m 1s
cargo-deny	23s
Install test (Linux)	1m 17s
Install test (macOS)	1m 58s
Install test (Windows)	2m 10s
Test Publish	51s
Test (ubuntu-latest)	15s
Test (windows-latest)	2m 21s
Test (macOS-latest)	31s
	16m 8s

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