Disable security warnings from MSVC in tests

MSVC generates the following for those tests:
warning C4996: '_open': This function or variable may be unsafe. Consider using _sopen_s
instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS. See online
help for details.

We need to set the define before any C runtime headers are included, so
do it right away, it doesn't do any harm on other compilers.

Change-Id: Ia25afb87934058c3f27e63820eeb2db063a627f1
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

tests/auto/corelib/io/qfile/tst_qfile.cpp

@@ -27,6 +27,8 @@
 **
 ****************************************************************************/
 
+#define _CRT_SECURE_NO_WARNINGS 1
+
 #include <QTest>
 #include <QScopedValueRollback>
 #include <qplatformdefs.h>

tests/auto/corelib/io/qipaddress/tst_qipaddress.cpp

@@ -26,6 +26,8 @@
 **
 ****************************************************************************/
 
+#define _CRT_SECURE_NO_WARNINGS 1
+
 #include <QtCore/QString>
 #include <QTest>
 #include <QtCore/private/qipaddress_p.h>