[Chore]: add integration test for EXTENSIONS_GALLERY

[jsjoeio]

Description

We were debugging an issue the other day and I thought EXTENSIONS_GALLERY wasn't working.

Solution

I'd like to add an integration test to ensure it does work

EXTENSIONS_GALLERY={} code-server --install-extension wesbos.theme-cobalt2
Installing extensions...
No extension gallery service configured.

Questions

if we can actually disable external connections it would be even better

This might be a good solution: https://stackoverflow.com/a/69727381/3015595