test: fix test runner on MinGW

MinGW does not export the _set_fmode function via its
libmsvcrt.a import library unless on an ARM platform. This
causes the test target build to fail without manually
adjusting the link parameters.

It is safe to assume that _fmode is available in all stable
releases, and it should be preferred unless using MSVC. This
is unrelated to #2407, but when both are fixed, MinGW builds
should complete successfully.

PR-URL: #2411
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

test/runner-win.c

@@ -55,7 +55,12 @@ int platform_init(int argc, char **argv) {
   _setmode(0, _O_BINARY);
   _setmode(1, _O_BINARY);
   _setmode(2, _O_BINARY);
+
+#ifdef _MSC_VER
   _set_fmode(_O_BINARY);
+#else
+  _fmode = _O_BINARY;
+#endif
 
   /* Disable stdio output buffering. */
   setvbuf(stdout, NULL, _IONBF, 0);