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-shiftwidth-zero.vim

@@ -0,0 +1,11 @@
+" shiftwidth=0 uses the value of tabstop, same as shiftwidth=4 here
+set shiftwidth=0 tabstop=4
+let g:vim_vue_plugin_config = {'foldexpr': 1}
+let g:test_flatten = 1
+
+function! Check()
+  let levels = [1, 2, 3, 2, 3, 4, 3, 3, 2, 1]
+  for lnum in range(1, len(levels))
+    call ExpectFoldlevel(lnum, levels[lnum - 1])
+  endfor
+endfunction