upgrade: fix generated sequence upgrade from 2.1

The bug was in an attempt to update a record in _space_sequence
in-place, to add field path and number. This was not properly
supported by the system space's trigger, and was banned in the
previous patch of this series.

But delete + tuple update + insert work fine. The patch uses them.

To test it the old disabled and heavily outdated
xlog/upgrade.test.lua was replaced with a smaller analogue, which
is supposed to be created separately for each upgrade bug.
According to the new policy of creating test files.

The patch tries to make it easy to add new upgrade tests and
snapshots. A new test should consist of fill.lua script to
populate spaces, snapshot, needed xlogs, and a .test.lua file.
Fill script and binaries should be in the same folder as test file
name, which is located in version folder. Like this:

 xlog/
 |
 + <test_name>.test.lua
 |
 +- upgrade/
    |
    +- <version>/
    |   |
    |   +-<test_name>/
    |     |
    |     +- fill.lua
    |     +- *.snap
    |     +- *.xlog

Version is supposed to say explicitly what a version files in
there have.

Closes #4771