Add experimental builds to CI (#269)

Author: johnnyshields

.github/workflows/test.yml

@@ -35,9 +35,18 @@ jobs:
           - ruby: 2.3
             mongoid: 6
             experimental: false
+          - ruby: 2.7
+            mongoid: HEAD
+            experimental: true
+          - ruby: 3.0
+            mongoid: HEAD
+            experimental: true
           - ruby: head
             mongoid: 7
             experimental: true
+          - ruby: head
+            mongoid: HEAD
+            experimental: true
           - ruby: jruby-head
             mongoid: 7
             experimental: true