Add tests and GitHub Actions

Replace the Travis tests, which used a separate repo and no longer
ran, with tests in this repo. Each case in test/cases is indented as
a whole and should be unchanged, with optional syntax, fold and tag
checks. The cases are from the previous tests, plus script-setup.vue
for <script setup> keywords and a sync check in oneline.vim.

test/run.sh runs the cases with Vim in a pseudo terminal, as syntax in
Ex mode isn't the same, and Neovim headless.

test/cases/basic.vue

@@ -0,0 +1,20 @@
+<template>
+  <p>{{ greeting }}</p>
+</template>
+
+<script>
+export default {
+  data: function () {
+    return {
+      greeting: 'Hello'
+    };
+  },
+};
+</script>
+
+<style scoped>
+p {
+  font-size: 2em;
+  text-align: center;
+}
+</style>