fio: introduce utime function

Closes #4323

@TarantoolBot document
Title: fio.utime

fio.utime (filepath [, atime [, mtime]])
Set access and modification times of a file.
The first argument is the filename, the second argument (atime) is
the access time, and the third argument (mtime) is
the modification time. Both times are provided in seconds since the epoch.
If the modification time is omitted, the access time provided is used;
if both times are omitted, the current time is used.