test: reproduce playground not returning alerts (#878)

test/blackbox-tests/melange-playground/ppx-alerts.t

@@ -0,0 +1,54 @@
+  $ cat > input.js <<EOF
+  > require(process.env.DUNE_SOURCEROOT + '/_build/default/bin/jsoo_main.bc.js');
+  > require(process.env.DUNE_SOURCEROOT + '/_build/default/bin/melange-cmijs.js');
+  > console.log(ocaml.compileRE('external foo: (([@mel.uncurry] (unit => unit)) => [@mel.uncurry] (unit => unit)) => unit = "Array.from"'));
+  > EOF
+
+  $ node input.js
+  File "_none_", line 1, characters 18-29:
+  Alert unused: Unused attribute [@mel.uncurry]
+  This means such annotation is not annotated properly.
+  For example, some annotations are only meaningful in externals
+
+
+  File "_none_", line 1, characters 52-63:
+  Alert unused: Unused attribute [@mel.uncurry]
+  This means such annotation is not annotated properly.
+  For example, some annotations are only meaningful in externals
+
+  {
+    js_code: '// Generated by Melange\n' +
+      "/* This output is empty. Its source's type definitions, externals and/or unused code got optimized away. */\n",
+    warnings: [],
+    type_hints: [
+      { start: [Object], end: [Object], kind: 'core_type', hint: 'unit' },
+      { start: [Object], end: [Object], kind: 'core_type', hint: 'unit' },
+      { start: [Object], end: [Object], kind: 'core_type', hint: 'unit' },
+      {
+        start: [Object],
+        end: [Object],
+        kind: 'core_type',
+        hint: 'unit -> unit'
+      },
+      { start: [Object], end: [Object], kind: 'core_type', hint: 'unit' },
+      { start: [Object], end: [Object], kind: 'core_type', hint: 'unit' },
+      {
+        start: [Object],
+        end: [Object],
+        kind: 'core_type',
+        hint: 'unit -> unit'
+      },
+      {
+        start: [Object],
+        end: [Object],
+        kind: 'core_type',
+        hint: '(unit -> unit) -> unit -> unit'
+      },
+      {
+        start: [Object],
+        end: [Object],
+        kind: 'core_type',
+        hint: '((unit -> unit) -> unit -> unit) -> unit'
+      }
+    ]
+  }