bpo-45666: fixes warning with swprintf and %s (GH-29307)

Author: sobolevn

Misc/NEWS.d/next/Build/2021-10-29-12-54-53.bpo-45666.w2G63u.rst

@@ -0,0 +1 @@
+Fix warning of ``swprintf`` and ``%s`` usage in ``_testembed.c``

Programs/_testembed.c

@@ -1733,7 +1733,7 @@ static int check_use_frozen_modules(const char *rawval)
     if (rawval == NULL) {
         wcscpy(optval, L"frozen_modules");
     }
-    else if (swprintf(optval, 100, L"frozen_modules=%s", rawval) < 0) {
+    else if (swprintf(optval, 100, L"frozen_modules=%S", rawval) < 0) {
         error("rawval is too long");
         return -1;
     }