tests: Key regress.c's Windows portability macros on _WIN32

TEST_NULL_DEVICE, TEST_SETENV, TEST_UNSETENV, and TEST_MKDIR were
gated on USE_WINDOWS_API, while the arpa/inet.h/direct.h include
guard a few lines above keys on _WIN32. Whether mkdir(), setenv(),
and /dev/null work as expected is a libc availability question, not
a wolfSSH API-selection one, so _WIN32 is the more direct fit and
now both guards in this file agree.

No behavior change: USE_WINDOWS_API is only ever set when _WIN32 is
already defined, so this does not change which branch any current
build takes.

Verified with the MinGW cross compiler both with WOLFSSH_SFTP defined
and without, and confirmed a plain Linux build still passes
tests/regress.test and tests/unit.test.

tests/regress.c

@@ -9380,8 +9380,10 @@ static void TestClientBuffersIdempotent(void)
 }
 #endif
 
-/* Windows has no /dev/null; the null device there is "NUL". */
-#ifdef USE_WINDOWS_API
+/* Windows has no /dev/null; the null device there is "NUL". Keyed on
+ * _WIN32, like the arpa/inet.h/direct.h include guard above: this is a
+ * libc-availability question, not a wolfSSH API-selection one. */
+#ifdef _WIN32
     #define TEST_NULL_DEVICE   "NUL"
 #else
     #define TEST_NULL_DEVICE   "/dev/null"
@@ -13371,8 +13373,10 @@ static int KnownHostsCheckCapture(const byte* pubKey, word32 pubKeySz,
  * enough for this test's own HOME juggling. WMKDIR is not an option here:
  * it is only defined when wolfssh/port.h is built with SFTP, SCP, or sshd
  * support, and this test compiles whenever WOLFSSL_BASE64_ENCODE is set,
- * independent of those. */
-#ifdef USE_WINDOWS_API
+ * independent of those. Keyed on _WIN32 rather than USE_WINDOWS_API for
+ * the same reason as TEST_NULL_DEVICE above: mkdir()/setenv() availability
+ * is a libc question. */
+#ifdef _WIN32
     #define TEST_SETENV(n,v)   _putenv_s((n), (v))
     #define TEST_UNSETENV(n)   _putenv_s((n), "")
     #define TEST_MKDIR(p,m)    _mkdir((p))