Add CI tests for Go 1.9

Author: xdg

.github/workflows/test.yml

@@ -4,7 +4,7 @@ jobs:
   test:
     strategy:
       matrix:
-        go-version: [1.11.x, 1.12.x, 1.13.x, 1.14.x, 1.15.x, 1.16.x]
+        go-version: [1.9.x, 1.10.x, 1.11.x, 1.12.x, 1.13.x, 1.14.x, 1.15.x, 1.16.x]
         os: [ubuntu-latest]
     runs-on: ${{ matrix.os }}
     steps:
@@ -19,8 +19,6 @@ jobs:
         # In order:
         # * Module download cache
         # * Build cache (Linux)
-        # * Build cache (Mac)
-        # * Build cache (Windows)
         path: |
           ~/go/pkg/mod
           ~/.cache/go-build