Constantify serve_{dir,file} test file paths and content #1136

Job	Run time
check-docs	53s
cargo-hack	1m 27s
cargo-public-api-crates	1m 4s
check	54s
cargo-deny check (advisories)	21s
cargo-deny check (bans licenses sources)	22s
test-msrv	2m 28s
test-versions (stable)	1m 30s
style	11s
test-versions (beta)	1m 26s
test-versions (nightly)	1m 18s
	11m 54s

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