Assuming all your projects are written under Documents\CB, sorry but I don't see a clear solution. Maybe someone else might know. It's always just worked for me.
Maybe try adding that file specifically. I'm not sure if you can add a file if it doesn't already exist. But try adding the file you want to exclude directly, in advance before compiling it.
"C:\Users\[User name]\Documents\CB\[My Project]\bin\Debug\testfolder.exe"
Edit: I just thought of this. gcc/mingw might also be creating temporary files inside your %APPDATA%, or another temporary folder. I'm not sure exactly where those are...
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Also, it might if we can narrow down the issue.
First thing would be: Does the bin\Debug\testfolder.exe actually exist (regardless of the error)? Did you check?
If it does exit, it might be because your anti-virus or another program is holding onto the file, preventing you from modifying it.
https://stackoverflow.com/questions/6875403/cannot-open-output-file-permission-denied/6875488
If it doesn't, is that .exe showing up in quarantined items in your anti-virus?