test: Repro 'dune fmt' crash in presence of Nix result

Signed-off-by: Jules Aguillon <jules@j3s.fr>

test/blackbox-tests/test-cases/nix-result-in-tree.t

@@ -0,0 +1,58 @@
+Nix can leave a symlink to a store path in the tree, often called 'result'.
+'dune fmt' crashes because of that.
+
+  $ RESULT=`mktemp -d`
+  $ echo "let x  = 2" > "$RESULT/foo.ml"
+  $ chmod -R 555 "$RESULT"
+
+  $ ln -s "$RESULT" result
+  $ echo "(lang dune 2.8)" > dune-project
+  $ touch .ocamlformat
+
+  $ dune fmt --auto-promote
+  File "result/foo.ml", line 1, characters 0-0:
+  Error: Files _build/default/result/foo.ml and
+  _build/default/result/.formatted/foo.ml differ.
+  Promoting _build/default/result/.formatted/foo.ml to result/foo.ml.
+  Internal error, please report upstream including the contents of _build/log.
+  Description:
+    ("hooks failed",
+     { exns =
+         [ { exn = "Unix.Unix_error(Unix.EACCES, \"open\", \"result/foo.ml\")"
+           ; backtrace =
+               "Raised by primitive operation at Stdune__Io.Copyfile.sendfile_with_fallback.setup_copy in file \"otherlibs/stdune/src/io.ml\", line 142, characters 18-86\n\
+                Re-raised at Stdune__Exn.raise_with_backtrace in file \"otherlibs/stdune/src/exn.ml\", line 38, characters 27-56\n\
+                Called from Promote__Diff_promotion.do_promote.promote_one in file \"src/promote/diff_promotion.ml\", line 150, characters 6-40\n\
+                Called from Stdlib__Map.Make.iter in file \"map.ml\", line 305, characters 20-25\n\
+                Called from Promote__Diff_promotion.do_promote in file \"src/promote/diff_promotion.ml\", line 159, characters 4-51\n\
+                Called from Promote__Diff_promotion.finalize in file \"src/promote/diff_promotion.ml\", line 181, characters 28-51\n\
+                Called from Stdune__Exn_with_backtrace.try_with in file \"otherlibs/stdune/src/exn_with_backtrace.ml\", line 9, characters 8-12\n\
+                "
+           }
+         ]
+     })
+  Raised at Stdune__Code_error.raise in file
+    "otherlibs/stdune/src/code_error.ml", line 10, characters 30-62
+  Called from Dune__exe__Build_cmd.run_build_system.(fun) in file
+    "bin/build_cmd.ml", line 81, characters 6-31
+  Called from Fiber__Scheduler.exec in file "vendor/fiber/src/scheduler.ml",
+    line 76, characters 8-11
+  Re-raised at Stdune__Exn.raise_with_backtrace in file
+    "otherlibs/stdune/src/exn.ml", line 38, characters 27-56
+  Called from Fiber__Scheduler.exec in file "vendor/fiber/src/scheduler.ml",
+    line 76, characters 8-11
+  Re-raised at Stdune__Exn.raise_with_backtrace in file
+    "otherlibs/stdune/src/exn.ml", line 38, characters 27-56
+  Called from Fiber__Scheduler.exec in file "vendor/fiber/src/scheduler.ml",
+    line 76, characters 8-11
+
+  I must not crash.  Uncertainty is the mind-killer. Exceptions are the
+  little-death that brings total obliteration.  I will fully express my cases. 
+  Execution will pass over me and through me.  And when it has gone past, I
+  will unwind the stack along its path.  Where the cases are handled there will
+  be nothing.  Only I will remain.
+  [1]
+
+Allow Dune to remove temporary files (Dune crashes without this):
+
+  $ chmod -R u+w "$RESULT"