Merge #6111 from ZyX-I/split-eval'/os-fileio

Refactor writefile() and create more tests for it

test/functional/helpers.lua

@@ -344,6 +344,16 @@ local function write_file(name, text, dont_dedent)
   file:close()
 end
 
+local function read_file(name)
+  local file = io.open(name, 'r')
+  if not file then
+    return nil
+  end
+  local ret = file:read('*a')
+  file:close()
+  return ret
+end
+
 local function source(code)
   local fname = tmpname()
   write_file(fname, code)
@@ -585,6 +595,7 @@ local M = {
   sleep = sleep,
   set_session = set_session,
   write_file = write_file,
+  read_file = read_file,
   os_name = os_name,
   rmdir = rmdir,
   mkdir = lfs.mkdir,