test(module-compilation): show missing location snippet for ppxed files (#12770)

Signed-off-by: Antonio Nuno Monteiro <[email protected]>

Author: anmonteiro

test/blackbox-tests/test-cases/melange/ppx-preview.t

@@ -0,0 +1,40 @@
+Show PPX snippet preview is shown in Dune
+
+  $ mkdir -p lib
+  $ cat > dune-project <<EOF
+  > (lang dune 3.13)
+  > (using melange 0.1)
+  > EOF
+  $ cat > lib/dune <<EOF
+  > (library
+  >  (name the_lib)
+  >  (modes melange)
+  >  (preprocess (pps melange.ppx)))
+  > EOF
+  $ cat > lib/the_lib.ml <<EOF
+  > let x: nope = "hello"
+  > EOF
+
+  $ cat > dune <<EOF
+  > (melange.emit
+  >  (target output)
+  >  (libraries the_lib)
+  >  (emit_stdlib false))
+  > EOF
+
+  $ export DUNE_SANDBOX=symlink
+  $ dune build @melange
+  File "lib/the_lib.ml", line 1, characters 7-11:
+  Error: Unbound type constructor nope
+  [1]
+
+Works if the sandbox is disabled
+
+  $ export DUNE_SANDBOX=none
+  $ dune build @melange
+  File "lib/the_lib.ml", line 1, characters 7-11:
+  1 | let x: nope = "hello"
+             ^^^^
+  Error: Unbound type constructor nope
+  [1]
+