use sh instead of bash to run tests

Change-Id: Ie70eed7575b3c0cf515dd2589272f7317734ebee
Reviewed-on: https://code-review.googlesource.com/5350
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/runtests b/runtests
index aadcb92..ce7c18d 100755
--- a/runtests
+++ b/runtests
@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/usr/bin/env sh
 
 success=true
 for i