Constantify serve_{dir,file} test file paths and content #1137

Job	Run time
cargo-deny check (advisories)	21s
cargo-deny check (bans licenses sources)	19s
cargo-public-api-crates	24s
cargo-hack	1m 34s
check	58s
check-docs	55s
test-msrv	2m 37s
style	15s
test-versions (stable)	1m 26s
test-versions (beta)	1m 31s
test-versions (nightly)	1m 14s
	11m 34s

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