Add schema test for depends_on (#3205)

pipeline/frontend/yaml/linter/schema/.woodpecker/test-dag.yaml

@@ -0,0 +1,22 @@
+steps:
+  first:
+    image: test
+
+  second:
+    depends_on: first
+    image: test
+
+  next:
+    image: test
+    depends_on:
+      - first
+      - second
+
+  some:
+    image: test
+    depends_on:
+      - first
+
+  last:
+    image: test
+    depends_on: next

pipeline/frontend/yaml/linter/schema/schema_test.go

@@ -106,6 +106,11 @@ func TestSchema(t *testing.T) {
 			testFile: ".woodpecker/test-array-syntax.yaml",
 			fail:     false,
 		},
+		{
+			name:     "Step DAG syntax",
+			testFile: ".woodpecker/test-dag.yaml",
+			fail:     false,
+		},
 	}
 
 	for _, tt := range testTable {