tests: Add simple test for org.osuild.grub2.inst stage

Tests that grub2-mkimage ran without errors, and that the image file
actually has something written to it.

stages/test/test_grub2_inst.py

@@ -0,0 +1,33 @@
+#!/usr/bin/python3
+import os
+
+STAGE_NAME = "org.osbuild.grub2.inst"
+
+def test_grub2_partition(tmp_path, stage_module):
+    treedir = tmp_path / "tree"
+    os.makedirs(treedir, exist_ok=True)
+    with open(treedir / "disk.img", "wb") as f:
+        f.write(b"Just testing")
+
+    options = {
+        "filename": "disk.img",
+        "platform": "i386-pc",
+        "location": 0,
+        "core": {
+            "type": "mkimage",
+            "partlabel": "gpt",
+            "filesystem": "ext4",
+        },
+        "prefix": {
+            "type": "partition",
+            "partlabel": "gpt",
+            "number": 0,
+            "path": "/boot/",
+        },
+    }
+    stage_module.main(treedir, options)
+
+    # Make sure the test string is overwritten
+    with open(treedir / "disk.img", "rb") as f:
+        msg = f.read(12)
+        assert msg != b"Just testing"