Bug 10386: improvements to VirtualShelves.t

Most important: Does no longer delete all shelves!
Checks if there are ten borrowers for testing. But even works without them :)
When creating or modifying lists, takes name clashes into consideration.

Small change to _CheckShelfName in VirtualShelves module. Making it possible to
check a name for a list whose owner has been set to NULL. Note that a test
like field=? with undef for placeholder will not work in MySql.

Test plan:
How do you test a test? Well, you could run it on various databases..
But for real hacking, you could also add some debug lines.
I tested this by forcing 10 undefs in @borrowernumbers.
And by overwriting the return value of randomname with an existing name.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>