Do not assert when testing

openrct2.common.props

@@ -74,6 +74,16 @@
       <OptimizeReferences>true</OptimizeReferences>
     </Link>
   </ItemDefinitionGroup>
+  <ItemDefinitionGroup Condition="'$(Configuration)'=='DebugTests'">
+    <ClCompile>
+      <PreprocessorDefinitions>__TEST__;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+    </ClCompile>
+  </ItemDefinitionGroup>
+  <ItemDefinitionGroup Condition="'$(Configuration)'=='ReleaseTests'">
+    <ClCompile>
+      <PreprocessorDefinitions>NDEBUG;__TEST__;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+    </ClCompile>
+  </ItemDefinitionGroup>
 
   <!-- Debug|Win32 is reserved for RCT2 interop builds, this means it has to be a DLL -->
   <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'!='Debug|Win32'">

src/core/Guard.cpp

@@ -78,7 +78,9 @@ namespace Guard
             char *bufend = (char *)strchr(buffer, 0);
             vsnprintf(bufend, sizeof(buffer) - (bufend - buffer), message, args);
         }
-        int result = MessageBox(nullptr, buffer, OPENRCT2_NAME, MB_ABORTRETRYIGNORE | MB_ICONEXCLAMATION);
+        // Only show message box if we are not building for testing
+#ifndef __TEST__
+        int result = MessageBoxA(nullptr, buffer, OPENRCT2_NAME, MB_ABORTRETRYIGNORE | MB_ICONEXCLAMATION);
         if (result == IDABORT)
         {
 #ifdef USE_BREAKPAD
@@ -88,6 +90,7 @@ namespace Guard
             assert(false);
 #endif
         }
+#endif
 #else
         assert(false);
 #endif