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

Mainly a QoL change for downstream packagers, but might also prove
useful for e.g. future refactors.

Currently we have to patch all those file paths to something exsistent,
due to test-files/ not being included in crates.io releases, while the
Debian Rust Team pulls Rust crate source from there:

https://salsa.debian.org/rust-team/debcargo-conf/-/blob/ee75c0c18a/src/tower-http/debian/patches/missing-testdata

With these constants we could change only them.

Test files are also changed to the same content, so one constant fits
all.

Author: nc7s

test-files/missing_precompressed.txt

@@ -1 +1 @@
-Test file!
+Test file

test-files/only_gzipped.txt.gz

@@ -1 +1 @@
-"This is a test file!"
+Test file