Execute (Assert b:vimtex.tex = latexmain.tex):
  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*')

Execute (Assert b:vimtex.tex = main_texroot.tex):
  call system('touch main_texroot.tex')
  call writefile(['%! TeX Root = main_texroot.tex'], 'test.tex')
  edit test.tex
  AssertEqual 'main_texroot.tex', fnamemodify(b:vimtex.tex, ':t')
  bdelete!
  call system('rm main_texroot.tex')