Replaced 9999 with 158, the correct constant for EDIT_EVENT_ADD_EVENT_ART, as defined in the official MusicBrainz Server source

[invo-coder19]

Abstract

Resolves a long-standing FIXME comment in the test SQL fixture by replacing
the placeholder edit type ID 9999 with the correct, verified value 158
(EDIT_EVENT_ADD_EVENT_ART), sourced from the official MusicBrainz Server
constants file.

Problem

In [test/add_data.sql], the INSERT INTO musicbrainz.edit statement used a
placeholder value of 9999 for the "add event artwork" edit type, with a
comment acknowledging that the real value was unknown at the time of writing:

[brainzbot]

Can one of the admins verify this patch?

[mwiencek]

@brainzbot ok to test

[mwiencek]

thanks, just fix the comment since EDIT_COVER_ART_ADD isn't the name used in MBS