[gnome-desktop-testing] Fix some clang warnings

[Colin Walters <walters src gnome org>]

commit ca6305a9e40c24f3bfe80c2a5216e4484f565f35
Author: Colin Walters <walters verbum org>
Date:   Tue Mar 6 15:14:13 2018 -0500

    Fix some clang warnings
    
    Caught these when I happened to have `CC=clang` due to some refactoring in the
    libostree CI tests.

 src/gnome-desktop-testing-runner.c |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)
---
diff --git a/src/gnome-desktop-testing-runner.c b/src/gnome-desktop-testing-runner.c
index 53663b8..07148ca 100644
--- a/src/gnome-desktop-testing-runner.c
+++ b/src/gnome-desktop-testing-runner.c
@@ -415,7 +415,6 @@ run_test_async (GdtrTest                *test,
 
   GError *local_error = NULL;
   GError **error = &local_error;
-  g_autofree char *testname = NULL;
   g_autofree char *test_tmpdir = NULL;
   g_autofree char *test_squashed_name = NULL;
   g_autofree char *test_tmpname = NULL;
@@ -456,7 +455,7 @@ run_test_async (GdtrTest                *test,
         {
           int errsv = errno;
           g_set_error (error, G_IO_ERROR, g_io_error_from_errno (errsv),
-                       g_strerror (errsv));
+                       "%s", g_strerror (errsv));
           goto out;
         }
     }