Add WaitFor() helper and use it in Button::Click unit test

This test also sporadically happens under AppVeyor, check if it happens
because we need to wait for some time before the event is processed.

tests/waitfor.h

@@ -12,6 +12,31 @@
 #include "wx/stopwatch.h"
 #include "wx/window.h"
 
+#include <functional>
+
+// Function used to wait until the given predicate becomes true or timeout
+// expires.
+inline bool
+WaitFor(const char* what, const std::function<bool ()>& pred, int timeout = 500)
+{
+    wxStopWatch sw;
+    for ( ;; )
+    {
+        wxYield();
+
+        if ( pred() )
+            break;
+
+        if ( sw.Time() > timeout )
+        {
+            WARN("Timed out waiting for " << what);
+            return false;
+        }
+    }
+
+    return true;
+}
+
 // Class used to check if we received the (first) paint event: this is
 // currently used under GTK only, as MSW doesn't seem to need to wait for the
 // things to work, while under Mac nothing works anyhow.