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.

test/cases/shared-no-syntax-file.vim

@@ -0,0 +1,15 @@
+" postcss has no syntax or indent file, toml has no indent file. They have
+" their blocks and keep indent
+let g:vim_vue_plugin_config = {
+      \'syntax': {
+      \   'template': ['html'],
+      \   'style': ['css', 'postcss'],
+      \   'i18n': ['toml'],
+      \},
+      \}
+
+function! Check()
+  call ExpectBlock('color: blue', 'style', 'postcss', 'postcssStyleBlock')
+  call ExpectBlock('hello', 'i18n', 'toml', 'tomlI18nBlock')
+  call ExpectIndentEachLine()
+endfunction