Add test for error case

Signed-off-by: Nicolás Ojeda Bär <n.oje.bar@gmail.com>

test/blackbox-tests/test-cases/exes-with-c/run.t

@@ -16,3 +16,22 @@
 
   $ _build/default/bb.exe
   B
+
+  $ mkdir err
+  $ touch err/foo.ml err/stubs.c
+  $ cat > err/dune << EOF
+  > (executable
+  >  (name foo)
+  >  (c_names stubs))
+  > EOF
+  $ dune build --root err @all
+  Entering directory 'err'
+  Info: Creating file dune-project with this contents:
+  | (lang dune 2.0)
+  File "dune", line 1, characters 0-41:
+  1 | (executable
+  2 |  (name foo)
+  3 |  (c_names stubs))
+  Error: Pure bytecode executables cannot contain C stubs.
+  Did you forget to add `(modes exe)'?
+  [1]