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/controls/buttontest.cpp

@@ -23,6 +23,7 @@
 // Get operator<<(wxSize) so that wxSize values are shown correctly in case of
 // a failure of a CHECK() involving them.
 #include "asserthelper.h"
+#include "waitfor.h"
 
 class ButtonTestCase
 {
@@ -64,7 +65,10 @@ TEST_CASE_METHOD(ButtonTestCase, "Button::Click", "[button]")
     wxYield();
 
     sim.MouseClick();
-    wxYield();
+
+    // At least under wxMSW calling wxYield() just once doesn't always work, so
+    // try for a while.
+    WaitFor("button to be clicked", [&]() { return clicked.GetCount() != 0; });
 
     CHECK( clicked.GetCount() == 1 );
 }

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.