Default to testing with clang.

GCC is deprecated, so why is it still the default testing toolchain?

Change-Id: I6540370e5917b5b7e74f2869b4ab8d177e4ef68f
diff --git a/tests/run-all.py b/tests/run-all.py
index d694d00..1f5ef08 100644
--- a/tests/run-all.py
+++ b/tests/run-all.py
@@ -210,8 +210,8 @@
             help=('Run tests against the specified platform version. Defaults '
                   'to the contents of APP_PLATFORM in jni/Application.mk'))
         self.add_argument(
-            '--toolchain', default='4.9', choices=('4.9', 'clang'),
-            help='Toolchain for building tests. Defaults to gcc-4.9.')
+            '--toolchain', default='clang', choices=('4.9', 'clang'),
+            help='Toolchain for building tests. Defaults to clang.')
 
         self.add_argument(
             '--show-commands', action='store_true',