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-multiline-tag.vim

@@ -0,0 +1,6 @@
+" Lines after a multiline start tag or empty tag ending with '>' or '/>'
+let g:test_flatten = 1
+
+function! Check()
+  call ExpectIndentEachLine()
+endfunction