Add tests for `serde` impl's `Visitor::expected` (#213) #437

Job	Run time
doc	14s
clippy	20s
coverage	21s
careful	56s
miri (s390x-unknown-linux-gnu)	1m 9s
miri (x86_64-unknown-linux-gnu)	54s
rustfmt	4s
test (1.85.0)	50s
build (stable, thumbv7em-none-eabi)	1m 14s
build (1.85.0, thumbv7em-none-eabi)	1m 27s
build (1.85.0, wasm32-unknown-unknown)	1m 20s
build (stable, armv7a-none-eabi)	1m 9s
build (stable, wasm32-unknown-unknown)	1m 16s
test (stable)	41s
build (1.85.0, armv7a-none-eabi)	1m 20s
	13m 15s

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