Make `black_box` a no-op #843

Job	Run time
Test (ubuntu-24.04)	9m 42s
Test (macOS-latest)	8m 52s
Test (windows-2022)	18m 12s
Compiletest (windows-2022)	4m 10s
Compiletest (ubuntu-24.04)	4m 7s
Difftest (macOS-latest)	15m 16s
Lint	6m 17s
Difftest (ubuntu-24.04)	13m 47s
Compiletest (macOS-latest)	3m 55s
Difftest (windows-2022)	28m 48s
Android (ubuntu-24.04, aarch64-linux-android)	8m 30s
cargo-deny	1m 14s
test_success	3s
	2h 2m 53s

Provide feedback