tst_ContainerApiSymmetry: fix mutable lambda anti-pattern

STL algorithms, in general, don't specify how often the function
objects passed to them are copied during the run of the
algorithm.

While generate_n is above any reasonable suspicion of copying the
function object after the first invocation, passing a mutable lambda
containing the counter is still an anti-pattern we don't want people
to copy.

Fix in the usual way, by keeping the counter external to the lambda.

As a drive-by, replace post- with pre-increment.

Amends dc091e7443.

Pick-to: 6.5 6.2
Change-Id: I9c44e769fd41e5f7157179a2be4c3534424cf913
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

tests/auto/corelib/tools/containerapisymmetry/tst_containerapisymmetry.cpp

@@ -720,7 +720,8 @@ Container make(int size)
     Container c;
     c.reserve(size);
     using V = typename Container::value_type;
-    std::generate_n(std::inserter(c, c.end()), size, [i = 1]() mutable { return V(i++); });
+    int i = 0;
+    std::generate_n(std::inserter(c, c.end()), size, [&i] { return V(++i); });
     return c;
 }