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/oneline.vim

@@ -0,0 +1,22 @@
+" One-line blocks like <style>p { color: red; }</style>
+
+function! Check()
+  call ExpectSyntax('script', 'javascriptScriptBlock')
+  call ExpectSyntax('p {', 'cssStyleBlock')
+  call ExpectSyntax('color', 'cssTextProp')
+  call ExpectSyntax('p {', 'cssStyleBlock')
+  call ExpectSyntax('color', 'cssTextProp')
+  call ExpectSyntax('Hello', 'htmlTemplateBlock')
+  call CheckSync()
+endfunction
+
+" Syntax from a sync point, as after jumping to a line not displayed yet
+function! CheckSync()
+  silent edit!
+  call cursor(line('$'), 1)
+  call search('Hello', 'cW')
+  if index(SyntaxList(), 'htmlTemplateBlock') == -1
+    call Fail('after syncing, Hello should be htmlTemplateBlock, but got '
+          \.string(SyntaxList()))
+  endif
+endfunction