tst_manualqdatastream: fix gcc error

GCC has a bug [1] that does not allow to specialize templates
outside of a namespace.
An attempt to build the test results in the following error:

 tst_manualqdatastream.cpp:18:15: error: explicit specialization in
 non-namespace scope ‘class tst_QDataStream’

Fix it by converting the template specializations into simple function
overloads.

[1]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85282

Pick-to: 6.7
Change-Id: I7803cce1687ca8ae992f1a57394346eb6fc90c8b
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

tests/manual/corelib/qdatastream/tst_manualqdatastream.cpp

@@ -15,11 +15,9 @@ class tst_QDataStream : public QObject
 
     template <class T>
     void fill(T &input);
-    template <>
+
     void fill(QSet<qsizetype> &input);
-    template <>
     void fill(QMap<qsizetype, qsizetype> &input);
-    template <>
     void fill(QHash<qsizetype, qsizetype> &input);
 
     template <class T>
@@ -58,7 +56,6 @@ void tst_QDataStream::fill(T &input)
     qDebug("Created dataset in %lld ms", timer.elapsed());
 }
 
-template <>
 void tst_QDataStream::fill(QSet<qsizetype> &input)
 {
     constexpr qsizetype GiB = 1024 * 1024 * 1024;
@@ -76,7 +73,6 @@ void tst_QDataStream::fill(QSet<qsizetype> &input)
     qDebug("Created dataset in %lld ms", timer.elapsed());
 }
 
-template <>
 void tst_QDataStream::fill(QMap<qsizetype, qsizetype> &input)
 {
     constexpr qsizetype GiB = 1024 * 1024 * 1024;
@@ -93,7 +89,6 @@ void tst_QDataStream::fill(QMap<qsizetype, qsizetype> &input)
     qDebug("Created dataset in %lld ms", timer.elapsed());
 }
 
-template <>
 void tst_QDataStream::fill(QHash<qsizetype, qsizetype> &input)
 {
     constexpr qsizetype GiB = 1024 * 1024 * 1024;