win32: fix unit tests (bug #53396)

Unit tests would not build on win32 since cc.h is taken from win32 port, but
sys_arch.c is specific to the unit tests. This results in the unit tests not
having a definition of lwip_win32_platform_diag

We resolve this by guarding the define with LWIP_TESTMODE, which is not
defined for lwIP_Test, but somehow defined for lwIP_Unittests (opt.h must
be included first someplace in the include tree)

ports/win32/include/arch/cc.h

@@ -87,8 +87,10 @@ typedef int sys_prot_t;
  * C standard lwIP targets does not support this in macros, we have extra brackets
  * around the arguments, which are left out in the following macro definition:
  */
+#if !defined(LWIP_TESTMODE) || !LWIP_TESTMODE
 void lwip_win32_platform_diag(const char *format, ...);
 #define LWIP_PLATFORM_DIAG(x) lwip_win32_platform_diag x
+#endif
 
 #ifndef LWIP_NORAND
 extern unsigned int sys_win_rand(void);