[stable20] fix sharer flag on ldap:show-remnants when user owned more than a single share #23702
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Little sibling of #23700 for Nextcloud 20 and below.
Fixes the condition and stupid DB query. But also adds a unit tests which requires the business logic being drawn out of OfflineUser's constructor, which was bad style anyhow.
Previously, if you had a gone LDAP user owning two or more shares, the "Sharer" flag in the table output would say N(o) instead of Y(es). Unit test added.