libtest: add a sleep macro for Windows

.. because sleep() is used in some libtests.

Closes https://github.com/curl/curl/pull/10295

tests/libtest/lib1515.c

@@ -37,10 +37,6 @@
 
 #define DNS_TIMEOUT 1
 
-#if defined(WIN32) || defined(_WIN32)
-#define sleep(sec) Sleep ((sec)*1000)
-#endif
-
 static int debug_callback(CURL *curl, curl_infotype info, char *msg,
                           size_t len, void *ptr)
 {

tests/libtest/lib1542.c

@@ -36,10 +36,6 @@
 #include "warnless.h"
 #include "memdebug.h"
 
-#if defined(WIN32) || defined(_WIN32)
-#define sleep(sec) Sleep ((sec)*1000)
-#endif
-
 int test(char *URL)
 {
   CURL *easy = NULL;

tests/libtest/test.h

@@ -42,6 +42,10 @@
 
 #include "curl_printf.h"
 
+#ifdef WIN32
+#define sleep(sec) Sleep ((sec)*1000)
+#endif
+
 #define test_setopt(A,B,C)                                      \
   if((res = curl_easy_setopt((A), (B), (C))) != CURLE_OK)       \
     goto test_cleanup