Print the name of the auto-detected gn build when running tests

Both x64.optdebug and x64.debug map to the x64.debug config, so
just printing arch/mode is still confusing.

R=machenbach@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2299623002
Cr-Commit-Position: refs/heads/master@{#39047}

tools/run-tests.py

@@ -443,6 +443,7 @@ def ProcessOptions(options):
         latest_timestamp = os.path.getmtime(gn_config_dir)
         latest_config = gn_config
     if latest_config:
+      print(">>> Latest GN build found is %s" % latest_config)
       options.outdir = os.path.join(DEFAULT_OUT_GN, latest_config)
 
   build_config_path = os.path.join(