test/buildroot: check BuildRoot.output

Now that the `org.osbuild.linux` runner does not use `api.setup_stdio`
anymore, the output of the binary run from the BuildRoot must end up
in `BuildRoot.output`. Check for that.

test/mod/test_buildroot.py

@@ -71,6 +71,24 @@ class TestBuildRoot(test.TestBase):
         assert r.output
         self.assertEqual(log, r.output)
 
+    def test_output(self):
+        runner = "org.osbuild.linux"
+        libdir = os.path.abspath(os.curdir)
+        var = pathlib.Path(self.tmp.name, "var")
+        var.mkdir()
+
+        data = "42. cats are superior to dogs"
+
+        monitor = NullMonitor(sys.stderr.fileno())
+        with BuildRoot("/", runner, libdir=libdir, var=var) as root:
+            api = osbuild.api.API({}, monitor)
+            root.register_api(api)
+
+            r = root.run(["/usr/bin/echo", data], monitor)
+            self.assertEqual(r.returncode, 0)
+
+        self.assertEqual(data, r.output.strip())
+
     @unittest.skipUnless(test.TestBase.have_test_data(), "no test-data access")
     def test_bind_mounts(self):
         runner = "org.osbuild.linux"