Return non-zero exit code on the GUI test failure

The main reason to return EXIT_FAILURE if the test was failed is showing
the right status of the test in GitHub CI checks.

main_wx_test.cpp

@@ -175,6 +175,9 @@ class application_test final
     // Used to check if distribution tests should be enabled.
     bool is_distribution_test() const { return is_distribution_test_; }
 
+    // Returns the exit code based of tests results.
+    int get_exit_code() const { return exit_code_; }
+
   private:
     application_test() = default;
     application_test(application_test const&) = delete;
@@ -231,6 +234,8 @@ class application_test final
     bool run_all_              {true};
 
     bool is_distribution_test_ {false};
+
+    int  exit_code_            {EXIT_FAILURE};
 };
 
 application_test& application_test::instance()
@@ -509,6 +514,8 @@ TestsResults application_test::run()
             }
         }
 
+    exit_code_ = results.failed == 0 ? EXIT_SUCCESS : EXIT_FAILURE;
+
     return results;
 }
 
@@ -645,6 +652,7 @@ class SkeletonTest final : public Skeleton
 
     // wxApp overrides.
     bool OnInit                 () override;
+    int  OnRun                  () override;
     bool OnExceptionInMainLoop  () override;
     bool StoreCurrentException  () override;
     void RethrowStoredException () override;
@@ -711,6 +719,20 @@ bool SkeletonTest::OnInit()
     return true;
 }
 
+int SkeletonTest::OnRun()
+{
+    int exit_code = Skeleton::OnRun();
+
+    // If the application exited successfully then return the exit code
+    // based on test results.
+    if(exit_code == 0)
+        {
+        exit_code = application_test::instance().get_exit_code();
+        }
+
+    return exit_code;
+}
+
 bool SkeletonTest::StoreCurrentException()
 {
     // We store all the exceptions that are expected to be caught by the tests