QtTest: port approx_wide_len() from std::clamp() to qt_saturate()

The patch that originally added the function used std::clamp() because
qt_saturate() wasn't available in all the branches to which the patch
was backported. This patch modernizes the code for newer branches.

Pick-to: 6.7 6.5
Change-Id: I1b764d303e00ec04858643efed3dcc71f2c7ce4c
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit d042c14c9940c6aabaa5b39d3061c0f9e50fff06)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/testlib/qtestresult.cpp

@@ -12,7 +12,8 @@
 #include <QtTest/qtestassert.h>
 #include <QtTest/qtesteventloop.h>
 
-#include <algorithm>
+#include <QtCore/private/qnumeric_p.h>
+
 #include <climits>
 #include <cwchar>
 
@@ -334,11 +335,10 @@ static int approx_wide_len(const char *s)
     std::mbstate_t state = {};
     // QNX might stop at max when dst == nullptr, so pass INT_MAX,
     // being the largest value this function will return:
-    constexpr size_t max = INT_MAX;
-    auto r = std::mbsrtowcs(nullptr, &s, max, &state);
+    auto r = std::mbsrtowcs(nullptr, &s, INT_MAX, &state);
     if (r == size_t(-1)) // encoding error, fall back to strlen()
         r = strlen(s); // `s` was not advanced since `dst == nullptr`
-    return int(std::clamp(r, size_t(0), max));
+    return qt_saturate<int>(r);
 }
 
 // Overload to format failures for "const char *" - no need to strdup().