chore(developer): enable vscode test on Linux

[srl295]

• in feat(developer): scaffolding for Keyman Developer for VSCode🗼  #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.