add workflow for vim9

.github/workflows/test-vim9.yaml

@@ -0,0 +1,38 @@
+name: test-vim9
+
+on:
+  push:
+    branches: [vim9]
+    tags:
+      - v*.*.*
+    paths:
+      - '!README.md'
+      - '!.gitignore'
+      - '!LICENSE'
+      - '!doc/**'
+      - '.github/**'
+      - 'autoload/**'
+      - 'tests/**'
+      - 'examples/**'
+
+  pull_request:
+    branches: [vim9]
+
+jobs:
+  test-on-Ubuntu:
+    runs-on: ubuntu-20.04
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Build latest Vim
+        run: |
+          git clone https://github.com/vim/vim.git --depth=1 -j 4
+          cd vim
+          mkdir $HOME/local
+          ./configure --prefix=$HOME/local
+          make -j4
+          make install
+
+      - name: Test
+        run: $HOME/local/bin/vim -S tests/test.vim ; cat test.log