Using NUnit Console Runner on mono, vim errorformat was not showing matches. This was my way around it.

OmniSharp.vim

function! OmniSharp#RunTests(mode) abort
  wall
  python buildcommand()

  if a:mode !=# 'last'
    python getTestCommand()
  endif

  let s:cmdheight=&cmdheight
  set cmdheight=5
  let b:dispatch = b:buildcommand . ' && ' . s:testcommand
  if executable('sed')
    " don't match on <filename unknown>:0
    " convert nunit console runner output to same format as used from xbuild
    let b:dispatch .= ' | sed -e "s/:0//" -e "s/.*at \(.*\) in \(.*\):\([[:digit:]]\+\)/\2(\3,0): \1/"'
  endif
  let &l:makeprg=b:dispatch
  "errorformat=msbuild,nunit stack trace
  setlocal errorformat=\ %#%f(%l\\\,%c):\ %m
  Make
  let &cmdheight = s:cmdheight
endfunction