functests: Test invalid behaviour

Test correctly fail for oneline ruby, python and python3.

test/functional/helpers.lua

@@ -566,6 +566,19 @@ local function get_pathsep()
   return funcs.fnamemodify('.', ':p'):sub(-1)
 end
 
+local function check_provider(provider)
+  if provider == 'ruby' then
+    local prog = funcs['provider#' .. provider .. '#Detect']()
+    return prog ~= ''
+  elseif provider == 'python' or provider == 'python3' then
+    local py_major_version = (provider == 'python3' and 3 or 2)
+    local errors = funcs['provider#pythonx#Detect'](py_major_version)[2]
+    return errors == ''
+  else
+    assert(false, 'Unknown provider: ' .. provider)
+  end
+end
+
 local module = {
   prepend_argv = prepend_argv,
   clear = clear,
@@ -632,6 +645,7 @@ local module = {
   meth_pcall = meth_pcall,
   NIL = mpack.NIL,
   get_pathsep = get_pathsep,
+  check_provider = check_provider,
 }
 
 return function(after_each)