tests/server: fix to check against winsock2 error codes on Windows

Windows's winsock2 returns socket errors via `WSAGetLastError()` and
not via `errno` like most systems out there. This was covered by
switching to the `SOCKERRNO` curl macro earlier. But, on Windows the
returned socket error codes have different values than the standard
POSIX errno values. Existing code was using the POSIX values for all
these checks. Meaning they never actually matched on Windows.

This patch defines a set of `SOCKERRNO` constants that map to the
correct socket error values for Windows and other platforms.

The reverse issue exists in core curl code, which redefines POSIX errno
values to winsock2 ones, breaking non-socket uses on Windows.

Cherry-picked from #15000
Follow-up to adcfd4fb3e #16553
Bug: https://github.com/curl/curl/pull/16553#issuecomment-2704679377

Closes #16612

tests/server/util.h

@@ -26,17 +26,21 @@
 #include "server_setup.h"
 
 #ifdef USE_WINSOCK
-/* errno.h values */
-#undef  EINTR
-#define EINTR    4
-#undef  EAGAIN
-#define EAGAIN  11
-#undef  ENOMEM
-#define ENOMEM  12
-#undef  EINVAL
-#define EINVAL  22
-#undef  ERANGE
-#define ERANGE  34
+#define SOCKEADDRINUSE   WSAEADDRINUSE
+#define SOCKECONNREFUSED WSAECONNREFUSED
+#define SOCKEINPROGRESS  WSAEINPROGRESS
+#define SOCKEINTR        WSAEINTR
+#define SOCKEINVAL       WSAEINVAL
+#define SOCKEISCONN      WSAEISCONN
+#define SOCKEWOULDBLOCK  WSAEWOULDBLOCK
+#else
+#define SOCKEADDRINUSE   EADDRINUSE
+#define SOCKECONNREFUSED ECONNREFUSED
+#define SOCKEINPROGRESS  EINPROGRESS
+#define SOCKEINTR        EINTR
+#define SOCKEINVAL       EINVAL
+#define SOCKEISCONN      EISCONN
+#define SOCKEWOULDBLOCK  EWOULDBLOCK
 #endif
 
 enum {