osbuild-image-tests: test all exported artifacts

Run all exported images through imageImage.

cmd/osbuild-image-tests/main_test.go

@@ -497,9 +497,10 @@ func runTestcase(t *testing.T, testcase testcaseStruct, store string) {
 	err = runOsbuild(testcase.Manifest, store, outputDirectory, exports)
 	require.NoError(t, err)
 
-	imagePath := fmt.Sprintf("%s/%s", outputDirectory, testcase.ComposeRequest.Filename)
-
-	testImage(t, testcase, imagePath)
+	for _, export := range exports {
+		imagePath := path.Join(outputDirectory, export, testcase.ComposeRequest.Filename)
+		testImage(t, testcase, imagePath)
+	}
 }
 
 // getAllCases returns paths to all testcases in the testcase directory