tst_QByteArrayView: remove use of std::char_traits

Complements ab05e05f60253b9be615c09aa340ee75f2e5bcaf:
    Strings: get rid of the last std::char_traits use

    LLVM's libc++ became pedantic and insists on only accepting
    instantiations for one of the standard Char types, so remove all uses of
    it with a template parameter.

    Complements dc2ae08e02730ab795445bc047221aa56914f723.

which is:

    QStringView: stop instantiating std::char_traits<QChar> and <uchar>

    It's deprecated and will be removed with LLVM 19.

    Amends b1ee49b46533d39f7fabda68d0bd08a1ab130a27.

Fixes: QTBUG-128325
Task-number: QTBUG-126214
Task-number: QTBUG-122753
Pick-to: 6.7
Change-Id: I2c7fbe3c0f557e98e7edfffdb00b4cc71a6612ca
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Cristian Adam <cristian.adam@qt.io>
(cherry picked from commit b387537d3e9c35d9f112d4aa32ad71df5e5e9028)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

tests/auto/corelib/text/qbytearrayview/tst_qbytearrayview.cpp

@@ -519,7 +519,7 @@ namespace help {
 template <typename T>
 size_t size(const T &t) { return size_t(t.size()); }
 template <typename T>
-size_t size(const T *t) { return std::char_traits<T>::length(t); }
+size_t size(const T *t) { return QtPrivate::lengthHelperPointer(t); }
 
 template <typename T>
 decltype(auto)             cbegin(const T &t) { return t.begin(); }