test: add cygwin feature and use it (test 1056, 1517)

To replace custom checks with the same effect.

Closes #18327

tests/libtest/lib1517.c

@@ -61,17 +61,6 @@ static CURLcode test_lib1517(const char *URL)
 
   struct t1517_WriteThis pooh;
 
-  if(!strcmp(URL, "check")) {
-#if (defined(_WIN32) || defined(__CYGWIN__))
-    curl_mprintf("Windows TCP does not deliver response data but reports "
-                 "CONNABORTED\n");
-    return TEST_ERR_FAILURE; /* skip since it fails on Windows without
-                                workaround */
-#else
-    return CURLE_OK; /* sure, run this! */
-#endif
-  }
-
   pooh.readptr = testdata;
   pooh.sizeleft = strlen(testdata);