Execute (Find correct main file from subfiles):
  call writefile([
    \ '\documentclass{article}',
    \ '\usepackage{subfiles}',
    \ '\begin{document}',
    \ '\end{document}',
    \ '\subfile{test.tex}',
    \], 'main.tex')
  call writefile([
    \ '\documentclass[main.tex]{subfiles}',
    \ '\begin{document}',
    \ '\end{document}',
    \], 'subfile.tex')
  edit subfile.tex
  AssertEqual 'main.tex', fnamemodify(b:vimtex.tex, ':t')
  bdelete!

Execute (Clean up):
  call system('rm -f main.tex subfile.tex')