Tests: C++: Make tests run from shadow builds.

For shadow builds some tests couldn't find their test data. Solved by
using the same pattern as in Qt tests: Inject $$PWD as SRCDIR via a
DEFINES statement.

Note: It was considered to use QFINDTESTDATA for Qt5, but this was
rejected due to:
 1) It's Qt5 only (would mean to introduce some wrapper...)
 2) As the doc states, it will not work with QTEST_APPLESS_MAIN tests.

Change-Id: Ie6cf59570fe61725b7f6f83b7da650331ddb38fc
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

tests/auto/cplusplus/cxx11/tst_cxx11.cpp

@@ -56,7 +56,8 @@ class tst_cxx11: public QObject
     */
     static QString testdata(const QString &name = QString())
     {
-        static const QString dataDirectory = QDir::currentPath() + QLatin1String("/data");
+        static const QString dataDirectory = QLatin1String(SRCDIR "/data");
+
         QString result = dataDirectory;
         if (!name.isEmpty()) {
             result += QLatin1Char('/');