Before:
  Save $TMPDIR

After:
  Restore

Execute(ale#util#Tempname should create files in /tmp if $TMPDIR isn't set):
  if has('unix')
    let $TMPDIR = ''
    Assert ale#util#Tempname() =~# '^/tmp'
    " We should unlet the environment variable again.
    AssertEqual '', $TMPDIR
  endif