Show output of --list when it fails.

This should help to debug a CI problem, but is also likely to be
generally useful.

Change-Id: Ia7ff563548319258f0745924f19e62e887725c24
diff --git a/tools/test_runner.py b/tools/test_runner.py
index 6a7d8b4..3a3f459 100644
--- a/tools/test_runner.py
+++ b/tools/test_runner.py
@@ -35,8 +35,9 @@
 
 # Scan matching tests and return a test manifest.
 def GetTests(runner, filters = []):
-  rc, output = util.getstatusoutput(runner +  ' --list')
-  if rc != 0: util.abort('Failed to list all tests')
+  cmd = runner +  ' --list'
+  rc, output = util.getstatusoutput(cmd)
+  if rc != 0: util.abort("Failed to list all tests. Output of " + cmd + ":\n" + output)
 
   tests = output.split()
   for f in filters: