Add test for `as_flattened(_mut)` #406

Job	Run time
test (1.85.0)	49s
miri (x86_64-unknown-linux-gnu)	53s
test (stable)	44s
rustfmt	25s
clippy	21s
coverage	26s
build (1.85.0, armv7a-none-eabi)	1m 24s
careful	1m 4s
build (stable, armv7a-none-eabi)	1m 17s
build (stable, thumbv7em-none-eabi)	1m 8s
miri (s390x-unknown-linux-gnu)	1m 6s
doc	16s
build (stable, wasm32-unknown-unknown)	1m 10s
build (1.85.0, thumbv7em-none-eabi)	1m 18s
build (1.85.0, wasm32-unknown-unknown)	1m 26s
	13m 47s

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