How to invoke

vim -u NONE -c 'source test.vim'

result

Before Syntax * = /
:edit a.vim
Before FileType * = vim/vim
FileType vim on a.vim
Before Syntax * = vim/vim
After Syntax * = vim/vim
After FileType * = vim/vim
:edit xyzzy
:setfiletype vim
Before FileType * = vim/vim
FileType vim on xyzzy
Before FileType * = html/html
Before Syntax * = html/html
After Syntax * = html/html
After FileType * = html/html
Before Syntax * = html/vim
After Syntax * = html/vim
After FileType * = html/vim

test.vim

autocmd FileType vim
\   call add(g:message, 'FileType vim on '.expand('%'))
\ | if expand('%') ==# 'xyzzy'
\ |   setlocal filetype=html
\ | endif

test.vim

set nocompatible
let g:message = []

augroup testBefore
  autocmd FileType *
  \ call add(g:message, 'Before FileType * = '.&l:filetype.'/'.expand('<amatch>'))
  autocmd Syntax *
  \ call add(g:message, 'Before Syntax * = '.&l:filetype.'/'.expand('<amatch>'))
augroup END

syntax enable

augroup testAfter
  autocmd FileType *
  \ call add(g:message, 'After FileType * = '.&l:filetype.'/'.expand('<amatch>'))
  autocmd Syntax *
  \ call add(g:message, 'After Syntax * = '.&l:filetype.'/'.expand('<amatch>'))
augroup END


call add(g:message, ':edit a.vim')
edit a.vim
call add(g:message, ':edit xyzzy')
edit xyzzy
call add(g:message, ':setfiletype vim')
setfiletype vim

put =g:message