Fix "cannot open file" test (finally)

tests/test_OFStream.cpp

@@ -48,4 +48,11 @@ TEST_CASE("OFStream", "[language]")
 
     REQUIRE(not std::filesystem::exists(filename));
   }
+
+  SECTION("bad filename")
+  {
+    if (parallel::rank() == 0) {
+      REQUIRE_THROWS_WITH(OFStream{"/"}, "error: cannot create file /");
+    }
+  }
 }