tests: Suppress unnecessary output from find_alternative

This suppress the following output from find_alternative:
which: no apache2 in ($PATH)

1 files changed, 1 insertions, 1 deletions

diff --git a/test/gvfs-test b/test/gvfs-test
index 8a4ada1d..86ed6115 100755
--- a/test/gvfs-test
+++ b/test/gvfs-test
@@ -55,7 +55,7 @@ def find_alternative(cmds):
     '''Find command in cmds array and return the found alternative'''
 
     for cmd in cmds:
-        if subprocess.call(['which', cmd], stdout=subprocess.PIPE) == 0:
+        if subprocess.call(['which', cmd], stdout=subprocess.PIPE, stderr=subprocess.PIPE) == 0:
             return cmd
 
 in_testbed = os.path.exists('/home/gvfs_sandbox_marker')