feat: introduce `test-util` feature flag #981

Job	Run time
Test Suite	1m 43s
Check	1m 36s
Rustfmt	25s
Check rustdoc intra-doc links	35s
Clippy	1m 36s
Cross compile (wasm32-unknown-unknown)	14s
Cross compile (armv7-unknown-linux-gnueabihf)	17s
Cross compile (powerpc64-unknown-linux-gnu)	41s
Cross compile (powerpc-unknown-linux-gnu)	23s
	7m 30s

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