Write mkdirs in more idiomatic C++ style.

~ Rewrote mkdirs to be in C++ style.
~ Replaced adb_dir{start,stop} with std::string params and (r)find.
+ Added test for mkdirs.

Also make base/test_utils.h public and support temporary directories
as well as files.

Change-Id: I6fcbdc5e0099f3359d3aac6b00c436f250ca1329
diff --git a/base/logging_test.cpp b/base/logging_test.cpp
index c91857a..1a92c94 100644
--- a/base/logging_test.cpp
+++ b/base/logging_test.cpp
@@ -21,7 +21,7 @@
 
 #include "base/file.h"
 #include "base/stringprintf.h"
-#include "test_utils.h"
+#include "base/test_utils.h"
 
 #include <gtest/gtest.h>