Add shared test cases with vim-svelte-plugin

Name cases for scenarios also in vim-svelte-plugin as shared-*, adding
one for multiline tags. test/shared.sh lists shared cases in only one
of the plugins, as a warning in a separate job on GitHub Actions.

.github/workflows/test.yml

@@ -26,3 +26,11 @@ jobs:
         run: ./test/run.sh
         env:
           VUE_TEST_EDITORS: ${{ matrix.neovim && 'nvim' || 'vim' }}
+
+  shared:
+    name: shared cases with vim-svelte-plugin
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Check shared cases
+        run: ./test/shared.sh