Visual Studio Code test sidebar button disappeared

I restarted VSCode because there were some problems with it, and now the test button on the left, the one that looks like a flask, has disappeared! I tried to run the command "Test: Run All Tests", but I get a message "No tests found in this workspace. You may need to install a test provider extension". Did I just accidentally uninstall test support in VSCode??? I can still run pytest from the command line and that works fine.

EDIT

Please read my own answer below.