test(serde): add unit tests for serde optional #5105

Job	Run time
wasm-wasi	1m 20s
wasm-unknown	1m 13s
clippy	1m 4s
check no_std	31s
docs	56s
doctest	2m 18s
feature-checks	7m 37s
fmt	12s
test ubuntu-latest stable --no-default-features	1m 58s
test ubuntu-latest stable	1m 59s
test ubuntu-latest stable --all-features	2m 39s
test ubuntu-latest nightly --no-default-features	1m 21s
test ubuntu-latest nightly	1m 15s
test ubuntu-latest nightly --all-features	2m 28s
test ubuntu-latest 1.79 --no-default-features	1m 8s
test ubuntu-latest 1.79	1m 9s
test windows-latest stable --no-default-features	5m 38s
test windows-latest stable	4m 35s
test windows-latest stable --all-features	5m 46s
test windows-latest nightly --no-default-features	3m 28s
test windows-latest nightly	3m 21s
test windows-latest nightly --all-features	4m 37s
test windows-latest 1.79 --no-default-features	2m 39s
test windows-latest 1.79	2m 49s
ci success	4s
	1h 2m 5s

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