Expand `Buf` tests #771

Job	Run time
loom	18s
minrust	2m 13s
miri	17m 21s
cross (i686-unknown-linux-gnu)	25s
cross (armv7-unknown-linux-gnueabihf)	27s
rustfmt	6s
cross (powerpc-unknown-linux-gnu)	27s
stable (ubuntu-latest)	28s
cross (powerpc64-unknown-linux-gnu)	27s
stable (macos-latest)	24s
nightly	26s
cross (wasm32-unknown-unknown)	6s
stable (windows-latest)	1m 21s
tsan	1m 24s
Publish Documentation	0s
	25m 53s

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