Add red test

tests/AlpsProfileTest.php

@@ -41,6 +41,12 @@ public function testReadPhpInput(): void
         stream_wrapper_restore('php');
     }
 
+    public function testExternalRt(): void
+    {
+        $profile = new AlpsProfile(__DIR__ . '/Fake/alps.rt_external.json');
+        $this->assertSame('foo (safe)', (string) $profile->links['Index->Foo:foo']);
+    }
+
     public function testFileNotReadable(): void
     {
         $this->expectException(AlpsFileNotReadableException::class);

tests/Fake/alps.external.json

@@ -0,0 +1,12 @@
+{
+  "$schema": "https://alps-io.github.io/schemas/alps.json",
+  "alps": {
+    "descriptor": [
+      {"id": "Foo", "descriptor": [
+        {"href": "#id"}
+      ]
+      },
+      {"id": "id"}
+    ]
+  }
+}

tests/Fake/alps.rt_external.json

@@ -0,0 +1,11 @@
+{
+  "$schema": "https://alps-io.github.io/schemas/alps.json",
+  "alps": {
+    "descriptor": [
+      {"id": "Index", "descriptor": [
+        {"href": "#foo"}
+      ]},
+      {"id": "foo", "type": "safe", "rt": "Foo.json#Foo"}
+    ]
+  }
+}