test: sites modules substitution with dune install (ocaml#10326)

Repro for ocaml#10317

Signed-off-by: Etienne Millon <[email protected]>

otherlibs/dune-site/test/install-subst.t

@@ -0,0 +1,44 @@
+This checks that generated sites files correctly substituted with `dune install`.
+
+See #10317.
+
+  $ cat > dune-project << EOF
+  > (lang dune 3.0)
+  > (using dune_site 0.1)
+  > (package
+  >  (name a)
+  >  (sites (share data)))
+  > EOF
+
+  $ cat > dune << EOF
+  > (library
+  >  (public_name a)
+  >  (libraries dune-site))
+  > (generate_sites_module
+  >  (module s)
+  >  (sites a))
+  > EOF
+
+  $ dune build
+
+  $ check_placeholder() {
+  >   name=$1;
+  >   if grep -q DUNE_PLACEHOLDER "$name"; then
+  >     echo placeholder found in $name
+  >   fi
+  > }
+
+The generated module has placeholders.
+
+  $ check_placeholder _build/default/S.ml
+  placeholder found in _build/default/S.ml
+
+  $ dune install a --prefix out/ --datadir /nonexistent/data
+
+We expect no placeholders to be present after installation.
+
+  $ find out -type f | sort | while read f ; do check_placeholder $f ; done
+  placeholder found in out/lib/a/S.ml
+  placeholder found in out/lib/a/a.a
+  placeholder found in out/lib/a/a.cma
+  placeholder found in out/lib/a/a__S.cmt