test: missing opam file now causes build failure (#8662)

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

test/blackbox-tests/test-cases/missing-opam-generated-file.t

@@ -0,0 +1,21 @@
+This shows a difference since #8518: we set up a project with a generated opam
+file. If the resulting file is not present in the repository (or if it has been
+renamed to just `opam` as in this example), we now fail.
+
+A related issue happens when `pkg.opam` exists but is not up to date. The
+instruction in the opam file would previously consider the original version but
+will overwrite `pkg.opam`. Now it does not update the file.
+
+  $ cat > dune-project << EOF
+  > (lang dune 1.10)
+  > (generate_opam_files)
+  > (package
+  >  (name pkg))
+  > EOF
+
+  $ touch opam
+
+  $ dune build -p pkg @install
+  Error: No rule found for pkg.opam
+  -> required by alias install
+  [1]