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-undo.vim

@@ -0,0 +1,17 @@
+" Undo settings when filetype changes
+let g:vim_vue_plugin_config = {'foldexpr': 1}
+packadd matchit
+
+function! Check()
+  call ExpectEqual(&l:foldmethod, 'expr', 'foldmethod')
+  call ExpectEqual(&l:indentexpr, 'GetVueIndent()', 'indentexpr')
+  call ExpectEqual(exists('b:match_words'), 1, 'b:match_words')
+  setlocal filetype=text
+  call ExpectEqual(&l:foldmethod, 'manual', 'foldmethod after undo')
+  call ExpectEqual(&l:indentexpr, '', 'indentexpr after undo')
+  call ExpectEqual(exists('b:match_words'), 0, 'b:match_words after undo')
+  call ExpectEqual(exists('b:syng_str'), 0, 'b:syng_str after undo')
+  if !has('nvim')
+    call ExpectEqual(&l:matchpairs, &g:matchpairs, 'matchpairs after undo')
+  endif
+endfunction