Silence subtests in `Flag`s #717

Job	Run time
check rustfmt	6s
tests (ubuntu-latest, x86_64-unknown-linux-gnu)	3m 22s
check clippy	21s
build-std (ubuntu-latest, x86_64-unknown-linux-gnu)	1m 39s
tests (macos-latest, x86_64-apple-darwin)	2m 40s
build-std (macos-latest, x86_64-apple-darwin)	1m 21s
tests (windows-latest, i686-pc-windows-msvc)	5m 30s
build-std (windows-latest, i686-pc-windows-msvc)	2m 53s
	17m 52s

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