Expand `Buf` tests #773

Job	Run time
loom	30s
miri	16m 43s
cross (i686-unknown-linux-gnu)	29s
minrust	1m 46s
cross (armv7-unknown-linux-gnueabihf)	42s
nightly	1m 13s
cross (powerpc-unknown-linux-gnu)	24s
rustfmt	3s
cross (powerpc64-unknown-linux-gnu)	25s
cross (wasm32-unknown-unknown)	6s
stable (ubuntu-latest)	1m 35s
stable (macos-latest)	1m 29s
tsan	1m 25s
stable (windows-latest)	3m 42s
Publish Documentation	14s
	30m 46s

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