Make `black_box` a no-op #842

Job	Run time
Android (ubuntu-24.04, aarch64-linux-android)	8m 41s
Difftest (ubuntu-24.04)	13m 24s
Difftest (windows-2022)	25m 11s
Test (windows-2022)	17m 50s
Compiletest (ubuntu-24.04)	4m 9s
Difftest (macOS-latest)	14m 14s
Compiletest (macOS-latest)	4m 24s
Test (ubuntu-24.04)	9m 32s
Compiletest (windows-2022)	4m 19s
Test (macOS-latest)	14m 41s
cargo-deny	1m 33s
Lint	5m 58s
test_success	4s
	2h 4m 0s

Provide feedback