UnitTests: Add custom main that calls RegisterMsgAlertHandler

This prevents a failed assertion from hanging on the MSVC buildbots.

Source/UnitTests/UnitTestsMain.cpp

@@ -0,0 +1,30 @@
+// Copyright 2023 Dolphin Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+// Based on gtest_main.cc
+
+#include <cstdio>
+#include <fmt/format.h>
+
+#include "Common/MsgHandler.h"
+
+#include "gtest/gtest.h"
+
+namespace
+{
+bool TestMsgHandler(const char* caption, const char* text, bool yes_no, Common::MsgType style)
+{
+  fmt::print(stderr, "{}\n", text);
+  ADD_FAILURE();
+  // Return yes to any question (we don't need Dolphin to break on asserts)
+  return true;
+}
+}  // namespace
+
+int main(int argc, char** argv)
+{
+  fmt::print(stderr, "Running main() from UnitTestsMain.cpp\n");
+  Common::RegisterMsgAlertHandler(TestMsgHandler);
+
+  ::testing::InitGoogleTest(&argc, argv);
+  return RUN_ALL_TESTS();
+}