Add GitHub action workflow and update test outputs

[muzimuzhi]

Motivation

I'd like to contribute the removal of dependency on etexcmds. Having tests checked on OS different than mine would give me more confidence.

Description

• The action workflow allows being manually triggered (through on: workflow_dispatch), uses islandoftex's texlive docker image, and checks tests in testfiles (the one in testfiles-failed is not covered).
• The updates for test outputs mainly reflect the introduction of \stock(height|width) and changes in l3backend for color stack with XeTeX.

Update: I feel that the possibility of introducing workflow file is not high. 🤒