Make tests assume ANDROID_ROOT is not /system.

This is to enable better chroot-like testing.

Change-Id: I943c68b0ebcdab2f107f808b466ac694085255aa
diff --git a/runtime/utils_test.cc b/runtime/utils_test.cc
index ae24b77..d8f8950 100644
--- a/runtime/utils_test.cc
+++ b/runtime/utils_test.cc
@@ -384,7 +384,8 @@
 TEST_F(UtilsTest, ExecSuccess) {
   std::vector<std::string> command;
   if (kIsTargetBuild) {
-    command.push_back("/system/bin/id");
+    std::string android_root(GetAndroidRoot());
+    command.push_back(android_root + "/bin/id");
   } else {
     command.push_back("/usr/bin/id");
   }