chore(developer): enable vscode test on Linux 🗼

- in #12799 the vscode extension test was stubbed out of CI because the Linux builders don't have a X server all of the time

- linux/ibus-keyman/src/test/run-tests.sh solves this (thx @ermshiperete ) with

```
Xvfb -screen 0 1024x768x24 :33 &> /dev/null &
export DISPLAY=:33
```

We will want to bring this test back before merging #12759 to make sure we get a basic sanity check on the extension.