Expand `Buf` tests (#747) #803

Job	Run time
loom	29s
cross (i686-unknown-linux-gnu)	33s
miri	3h 39m 24s
minrust	1m 37s
cross (armv7-unknown-linux-gnueabihf)	33s
cross (powerpc-unknown-linux-gnu)	33s
nightly	1m 6s
panic=abort tests	41s
cross (powerpc64-unknown-linux-gnu)	34s
rustfmt	14s
cross (wasm32-unknown-unknown)	13s
stable (ubuntu-latest)	1m 27s
tsan	1m 30s
stable (macos-latest)	1m 25s
stable (windows-latest)	2m 29s
Publish Documentation	13s
	3h 53m 1s

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