Execute (Assert b:vimtex.tex = latexmain.tex):
  let g:tex_flavor = 'latex'
  call system('touch latexmain.tex')
  call system('touch latexmain.tex.latexmain')
  new test.tex
  AssertEqual 'latexmain.tex', fnamemodify(b:vimtex.tex, ':t')
  bdelete!
  call system('rm latexmain.tex*')