qtestcase.cpp: fix warning about ignoring the result of writev()

I don't get it on openSUSE Tumbleweed, but this is not the only place I
don't get a warn_unused_result warning and it happens elsewhere.

Change-Id: I7e305799c8594ebab255fffd16ee12ff30df6d4d
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/testlib/qtestcase.cpp

@@ -117,10 +117,10 @@ static struct iovec IoVec(const char *str)
     return r;
 }
 
-template <typename... Args> static void writeToStderr(Args &&... args)
+template <typename... Args> static ssize_t writeToStderr(Args &&... args)
 {
     struct iovec vec[] = { IoVec(std::forward<Args>(args))... };
-    ::writev(STDERR_FILENO, vec, std::size(vec));
+    return ::writev(STDERR_FILENO, vec, std::size(vec));
 }
 
 // async-signal-safe conversion from int to string