Add CI workflow to actually test-drive Vimux

.github/workflows/check.yml

@@ -0,0 +1,38 @@
+name: Check
+on: [push, pull_request]
+jobs:
+  check:
+    strategy:
+      fail-fast: false
+      matrix:
+        vimFlavor: ["vim", "nvim"]
+    runs-on: ubuntu-20.04
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Enable Universe package repository
+        run: |
+          sudo add-apt-repository universe
+          sudo apt-get update
+      - name: Install tmux and ${{ matrix.tagsProvider }}
+        run: |
+          sudo apt-get install tmux ${{ matrix.vimFlavor == 'nvim' && 'neovim' || 'vim' }}
+      - name: Review versions
+        run: |
+          tmux -V
+          ${{ matrix.vimFlavor }} --version
+      # This tests looks for two thigs:
+      # * That VIM doesn't hang. If it succedes it will quit quickly. If 5
+      #   seconds later the tmux session is still running either the runner pane
+      #   didn't get closed or (more likely) we threw some error and VIM is
+      #   sitting there expecting us to acknowledge the message(s).
+      # * That VIM exited normally. This check isn't very useful since :qa
+      #   never bubbles up an error, but if someday we use :cq for a test being
+      #   ready to check the exit code seems like a good thing.
+      - name: "Try Vimux"
+        run: |
+          ec="$(mktemp)"
+          tmux new -s ci -d "${{ matrix.vimFlavor == 'nvim' && 'nvim -u /dev/null --headless' || 'vim' }} -i NONE \"+so plugin/vimux.vim\" \"+VimuxRunCommand('date')\" \"+VimuxCloseRunner | qa\"; echo \$? > '$ec'"
+          sleep 5
+          tmux kill-session -t ci && exit 1
+          exit "$(cat $ec)"