unit-tests: Force a CET/CEST timezone Windows understands

As it is currently unclear what the "three-letter-timezone" for CEST is, we
use the German timezone, which actually is CET/CEST. SetEnvironmentVariable()
"TZ" does not seem to affect localtime(), so we use _putenv() instead.

src/libstrongswan/tests/suites/test_utils.c

@@ -701,7 +701,11 @@ Suite *utils_suite_create()
 	TCase *tc;
 
 	/* force a timezone to match non-UTC conversions */
+#ifdef WIN32
+	_putenv("TZ=GST-1GDT");
+#else
 	setenv("TZ", "Europe/Zurich", 1);
+#endif
 	tzset();
 
 	s = suite_create("utils");