Test all features

Add cases for attribute highlight on and off, block syntax by lang
and emmet-vim group names, OnChangeVueSyntax(), matchit, .wpy,
initial_indent as syntax and tag.syntax, and syntax after reloading.

Also check indenting a file after removing all indent with
g:test_flatten, which fails before the end tag fix as closing tags at
column 0 were taken as block end tags.

test/cases/miniprogram.vim

@@ -0,0 +1,9 @@
+" .wpy for miniprogram, with initial indent for script and style by default
+let g:test_flatten = 1
+
+function! Check()
+  call ExpectSyntax('view', 'VueCustomTag')
+  call ExpectSyntax('wx:if', 'VueKey')
+  call ExpectSyntax('show', 'VueExpression')
+  call ExpectSyntax('text', 'VueCustomTag')
+endfunction