crt: test: Replace usage of _beginthreadex() by CreateThread() in t_safe_flush.c test

t_safe_flush.c is spawning new thread via _beginthreadex() which only uses
WinAPI functions. So it does not need to use CRT function _beginthreadex()
for spawning thread and WinAPI CreateThread() is enough.

crtdll.dll does not contain _beginthreadex(), so this change allows to
compile t_safe_flush.c test against crtdll.dll.

Signed-off-by: LIU Hao <lh_mouse@126.com>