chore: remove `testnet` command #75

Job	Run time
Unused dependency check	48s
cargo-deny	25s
build (windows-latest, x86_64-pc-windows-msvc)	2m 44s
Clippy & fmt	16s
build (macos-latest, x86_64-apple-darwin)	1m 23s
Install test (Linux)	1m 0s
Install test (macOS)	1m 31s
build (ubuntu-latest, x86_64-unknown-linux-musl)	58s
Install test (Windows)	2m 3s
Test Publish	52s
Test (ubuntu-latest)	50s
Test (windows-latest)	2m 12s
Test (macOS-latest)	50s
	15m 52s

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