test: reproduce #10927 (#11027)

Demonstrate that $ dune subst doesn't work without a VCS repo

Signed-off-by: Rudi Grinberg <[email protected]>

Author: rgrinberg

test/blackbox-tests/test-cases/subst/without-repo.t

@@ -0,0 +1,13 @@
+Demonstrate $ dune subst without a git repository
+
+  $ cat > dune-project << EOF
+  > (lang dune 3.16)
+  > (name test)
+  > EOF
+
+  $ echo "%%NAME%%" > README.md
+
+  $ dune subst
+
+  $ cat README.md
+  %%NAME%%