Add Emacs HEAD to CI matrix

Author: purcell

.github/workflows/test.yml

@@ -22,7 +22,7 @@ jobs:
     strategy:
       matrix:
         os: [macos-13, ubuntu-latest, windows-latest]
-        emacs_version: ['26.3', '27.2', '28.2', '29.3']
+        emacs_version: ['26.3', '27.2', '28.2', '29.3', 'snapshot']
         java_version: ['11', '17']
         include:
           # aarch64 (macos-13 is Intel)