test: --ignore-promoted-rules and internal rules

reproduce #8417

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

<!-- ps-id: dc52a340-79a9-4d37-a486-0c0d168111d5 -->

test/blackbox-tests/test-cases/ignore-promoted-rules-internal-rules.t

@@ -0,0 +1,32 @@
+This test demonstrates the interaction between --ignore-promoted-rules and
+internal promote rules (like generating opam files)
+
+Reported in #8417
+
+  $ cat >dune-project <<EOF
+  > (lang 3.10)
+  > (generate_opam_files true)
+  > (package
+  >  (name foo))
+  > EOF
+
+  $ dune build foo.opam
+  File "dune-project", line 1, characters 0-11:
+  1 | (lang 3.10)
+      ^^^^^^^^^^^
+  Error: Invalid first line, expected: (lang <lang> <version>)
+  [1]
+  $ echo "extra" >> foo.opam
+  $ cat foo.opam
+  extra
+
+This should not modify the file now
+
+  $ dune build --ignore-promoted-rules foo.opam
+  File "dune-project", line 1, characters 0-11:
+  1 | (lang 3.10)
+      ^^^^^^^^^^^
+  Error: Invalid first line, expected: (lang <lang> <version>)
+  [1]
+  $ cat foo.opam
+  extra