serde: add unit tests for `OtherFields` #4982

Job	Run time
wasm-wasi	3m 40s
check no_std	59s
docs	1m 3s
clippy	1m 34s
doctest	2m 54s
feature-checks	8m 32s
fmt	15s
wasm-unknown	2m 38s
test ubuntu-latest stable --no-default-features	2m 30s
test ubuntu-latest stable	2m 35s
test ubuntu-latest stable --all-features	3m 22s
test ubuntu-latest nightly --no-default-features	1m 19s
test ubuntu-latest nightly	1m 22s
test ubuntu-latest nightly --all-features	2m 9s
test ubuntu-latest 1.79 --no-default-features	1m 41s
test ubuntu-latest 1.79	1m 43s
test windows-latest stable --no-default-features	4m 59s
test windows-latest stable	6m 57s
test windows-latest stable --all-features	7m 7s
test windows-latest nightly --no-default-features	3m 47s
test windows-latest nightly	3m 33s
test windows-latest nightly --all-features	4m 26s
test windows-latest 1.79 --no-default-features	4m 14s
test windows-latest 1.79	3m 53s
ci success	1s
	1h 17m 13s

Provide feedback