tst_QDir: fix -Wunused-result

We've been ignoring the result of symlink(2) and GCC 14 complained.

Fix by QCOMPARE()ing to 0, ie. verifying sucesss.

Amends 23d08ce2edab09562ad283dac5d46c09efec63ca.

Pick-to: 6.7 6.5 6.2 5.15
Change-Id: If58267e3883c77cd62aa72a42410c837b32d9c0b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit c880a3867a85c0efdc6773ad6b446a318198585a)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

tests/auto/corelib/io/qdir/tst_qdir.cpp

@@ -438,7 +438,7 @@ void tst_QDir::mkdirOnSymlink()
 
     // create our structure:
     dir.mkpath("two/three");
-    ::symlink("two/three", "symlink");
+    QCOMPARE(::symlink("two/three", "symlink"), 0);
 
     // try it:
     QString path = "symlink/../four/five";