libbase_test: win32: get some tests working

* Get it building on Win32 by implementing mkstemp() and mkdtemp().

* Run StringPrintf %z test on Windows because it passes because we build
with __USE_MINGW_ANSI_STDIO which implements %z.

Change-Id: Ia01f94e8258503381a1df6d3da6e40de59e57125
Signed-off-by: Spencer Low <CompareAndSwap@gmail.com>