chore: add gitattributes linguist-vendored for tests #4192

Job	Run time
check no_std	28s
check	17s
check no_std optimism,kzg-rs	31s
check serde	24s
check std	19s
clippy	1m 21s
docs	1m 16s
fmt	7s
test stable --no-default-features	1m 22s
TyposCheck	6s
test stable	58s
test stable --all-features	1m 38s
test beta --no-default-features	55s
test beta	1m 5s
test beta --all-features	1m 39s
test nightly --no-default-features	47s
test nightly	49s
test nightly --all-features	1m 31s
	15m 33s

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