Add zero_alloc test

tests/alloc/dune

@@ -0,0 +1,17 @@
+(executable
+ (name test)
+ (modes native)
+ (libraries landmark))
+
+(rule
+ (ignore-stderr
+  (with-stdout-to
+   test.out
+   (run ./test.exe))))
+
+(rule
+ (alias runtest)
+ (enabled_if
+  (>= %{ocaml_version} "4.04"))
+ (action
+  (diff test.out.expected test.out)))

tests/alloc/test.ml

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+module L = Landmark
+
+let name = "zero_alloc"
+let zero_alloc = L.register name
+
+let () =
+  L.start_profiling
+    ~profiling_options:{L.default_options with allocated_bytes = true} ();
+  L.enter zero_alloc;
+  L.exit zero_alloc
+
+let check_allocated_bytes () =
+  let open Landmark.Graph in
+
+  let graph = L.export () in
+  let node =
+    root graph
+    |> children graph
+    |> List.filter (fun n -> n.name = name)
+    |> List.hd
+  in
+  Printf.printf "Allocations:\n%d\n%d\n"
+    (node.allocated_bytes)
+    (node.allocated_bytes_major)
+
+let () =
+  check_allocated_bytes ()

tests/alloc/test.out.expected

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+Allocations:
+0
+0