tst_QSharedPointer: add more tests for forward-declared payloads

It is not sufficient that QSharedPointer<ForwardDeclared>'s default ctor
and dtor compile. Copy/move assignment/construction and swapping should work, too.

Arguably, there are more functions that should compile with just a forward-declared
payload, but this is a start.

Change-Id: I75470e3d4ba949c3e735c4078cbc123d53ec3007
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

tests/auto/corelib/tools/qsharedpointer/tst_qsharedpointer.cpp

@@ -397,6 +397,26 @@ void tst_QSharedPointer::useOfForwardDeclared()
 {
     // this just a compile test: use the forward-declared class
     QSharedPointer<ForwardDeclared> sp;
+
+    // copying should work, too:
+    QSharedPointer<ForwardDeclared> sp2 = sp;
+
+    // and assignment:
+    QSharedPointer<ForwardDeclared> sp3;
+    sp3 = sp;
+
+    // move assignment:
+    QSharedPointer<ForwardDeclared> sp4;
+    sp4 = qMove(sp);
+
+    // and move constuction:
+    QSharedPointer<ForwardDeclared> sp5 = qMove(sp2);
+
+    // swapping:
+    sp4.swap(sp3);
+    qSwap(sp4, sp3);
+
+    // and, of course, destruction
 }