Fix warning in recently-added testlib snippet

Amends commit fa296ee1dcf4c16fb6f242cf08949485e499fec3 to implement
MyObject::startup(), mark it void (its return was ignored) and flag
its static wrapper as [[maybe_unused]] to calm a compiler warning.

As a drive-by, fix indentation of the class definition.

Change-Id: I3918bf8a4625a7e2b4e6d4e0cbfa68a337115865
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/testlib/doc/snippets/code/src_corelib_kernel_qtestsupport_core.cpp

@@ -5,9 +5,9 @@
 // dummy class
 class MyObject
 {
-    public:
-        int isReady();
-        bool startup();
+public:
+    int isReady();
+    void startup() {}
 };
 
 // dummy function
@@ -27,7 +27,7 @@ int MyObject::isReady()
 return 1;
 }
 
-static bool startup()
+[[maybe_unused]] static bool startup()
 {
 //! [2]
     MyObject obj;