Disable tests using wxUIActionSimulator under non-MSW platforms

wxUIActionSimulator is just too unreliable to be used there, so while
fixing it should really be a priority, for now at least prevent these
spurious failures from masking any other ones, which indicate real
problems that need to be fixed.

Notice that these tests can still be enabled by setting the environment
variable WX_UI_TESTS to 1 (or disabled by setting it to 0 under MSW).

tests/test.cpp

@@ -360,6 +360,37 @@ extern bool IsAutomaticTest()
 
 #if wxUSE_GUI
 
+bool EnableUITests()
+{
+    static int s_enabled = -1;
+    if ( s_enabled == -1 )
+    {
+        // Allow explicitly configuring this via an environment variable under
+        // all platforms.
+        wxString enabled;
+        if ( wxGetEnv("WX_UI_TESTS", &enabled) )
+        {
+            if ( enabled == "1" )
+                s_enabled = 1;
+            else if ( enabled == "0" )
+                s_enabled = 0;
+            else
+                wxFprintf(stderr, "Unknown \"WX_UI_TESTS\" value \"%s\" ignored.\n", enabled);
+        }
+
+        if ( s_enabled == -1 )
+        {
+#ifdef __WXMSW__
+            s_enabled = 1;
+#else // !__WXMSW__
+            s_enabled = 0;
+#endif // __WXMSW__/!__WXMSW__
+        }
+    }
+
+    return s_enabled == 1;
+}
+
 void DeleteTestWindow(wxWindow* win)
 {
     if ( !win )