ci: Add experimental flag

Author: jcs090218

.github/workflows/test.yml

@@ -10,6 +10,7 @@ on:
 jobs:
   test:
     runs-on: ${{ matrix.os }}
+    continue-on-error: ${{ matrix.experimental }}
     strategy:
       fail-fast: false
       matrix:
@@ -18,7 +19,17 @@ jobs:
           - 26.3
           - 27.2
           - 28.2
-          - snapshot
+        experimental: [false]
+        include:
+        - os: ubuntu-latest
+          emacs-version: snapshot
+          experimental: true
+        - os: macos-latest
+          emacs-version: snapshot
+          experimental: true
+        - os: windows-latest
+          emacs-version: snapshot
+          experimental: true
 
     steps:
     - uses: actions/checkout@v3