split-attrs: improve the xdist error fix

[pp-mo]

See the problems + workaround reported here and here.
The current fix is really pretty ugly.
A proper 'fix' probably belongs in pytest or pytest-xdists,
but maybe a neater workaround would be conceivable?

[pp-mo]

( History : this was originally introduced at 481459c in #4960. )

But I think at some point since, this problem has fixed itself.
#5584

I think it was to fix this (at least partly), that the test numbering was introduced in lib/iris/tests/integration/test_netcdf__loadsaveattrs.py,
in the classes TestRoundtrip and TestLoad
-- but for some reason not in TestSave

IIRC the numbering also intends to align the TestRoundtrip and TestLoad testcases (though this may be a bit broken now)
In any case, no real need to remove the numbering now.

So I think this issue is good to close !