test: generate_opam_files and fresh build (#7546)

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

test/blackbox-tests/test-cases/dune-project-meta/fresh-build.t

@@ -0,0 +1,24 @@
+Building the .install file should not depend on foo.opam being present in the
+source tree if (generate_opam_files true) is enabled.
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.8)
+  > (package
+  >  (name foo))
+  > (generate_opam_files true)
+  > EOF
+
+  $ cat >dune <<EOF
+  > (install
+  >  (section share)
+  >  (files dune))
+  > EOF
+
+  $ dune build foo.install
+  $ grep opam _build/default/foo.install
+  [1]
+
+  $ dune build @check
+  $ dune build foo.install
+  $ grep opam _build/default/foo.install
+    "_build/install/default/lib/foo/opam"