test: add a repro case for #6607

Signed-off-by: Etienne Millon <me@emillon.org>

test/blackbox-tests/test-cases/dune

@@ -140,3 +140,9 @@
  (applies_to patch-back-source-tree)
  (enabled_if
   (<> %{system} macosx)))
+
+(cram
+ (applies_to github6607)
+ (deps
+  (package stdune)
+  (package ppx_expect)))

test/blackbox-tests/test-cases/github6607.t

@@ -0,0 +1,58 @@
+This shows that the test suite is impacted by CLICOLOR_FORCE=1 (#6607).
+This environment variable should only impact the output of dune, not behavior
+visible from within the test suite.
+
+  $ cat > dune-project << EOF
+  > (lang dune 1.0)
+  > EOF
+
+  $ cat > dune << EOF
+  > (library
+  >  (name l)
+  >  (libraries stdune)
+  >  (preprocess (pps ppx_expect))
+  >  (inline_tests))
+  > EOF
+
+  $ cat > l.ml << EOF
+  > open[@ocaml.alert "-unstable"] Stdune
+  > 
+  > let%expect_test _ =
+  >   User_message.print (User_error.make []);
+  >   [%expect]
+  > EOF
+
+When running with `CLICOLOR_FORCE=0`, no escape codes are present in the output
+string. But when it is set to `1`, escape codes are present.
+Remarks:
+- it is necessary to pipe into `tr` to see the difference because otherwise these
+escape codes are stripped by cram, and it is necessary
+- it is necessary to call `dune clean` because `dune` does not know that the
+runtest depends on `CLICOLOR_FORCE`.
+
+The expected outcome would be that both of these runs output the same corrected
+file.
+
+  $ CLICOLOR_FORCE=0 dune runtest --auto-promote
+  File "l.ml", line 1, characters 0-0:
+  Error: Files _build/default/l.ml and _build/default/l.ml.corrected differ.
+  Promoting _build/default/l.ml.corrected to l.ml.
+  [1]
+  $ < l.ml tr '\033' '?'
+  open[@ocaml.alert "-unstable"] Stdune
+
+  let%expect_test _ =
+    User_message.print (User_error.make []);
+    [%expect{| Error: |}]
+  $ dune clean
+  $ CLICOLOR_FORCE=1 dune runtest --auto-promote
+  File "l.ml", line 1, characters 0-0:
+  Error: Files _build/default/l.ml and _build/default/l.ml.corrected differ.
+  Promoting _build/default/l.ml.corrected to l.ml.
+  [1]
+  $ < l.ml tr '\033' '?'
+  open[@ocaml.alert "-unstable"] Stdune
+
+  let%expect_test _ =
+    User_message.print (User_error.make []);
+    [%expect{| ?[1;31mError?[0m: |}]