Fix core DB data in unit testing

Fix core DB data in unit testing

Iec4ed4c8 made it so addCoreDBData() only gets called once. Which is
nice, except for the fact that it means any test that puts
'user_groups' into $this->tablesUsed is going to cause UTSysop to no
longer be a sysop, so any later test that expects it to be one is going
to mysteriously fail.

The fix is to make resetDB() actually clear out the 'user' table when
requested, but then to re-call addCoreDBData() so UTSysop is still
there.

This might break extension tests if they're relying on the 'user' table
never being cleared even if it's specified in $this->tablesUsed, but
hopefully that's not often the case.

Bug: T132411
Change-Id: If251739fd486544f54a0c07edcc24aeef0998342
Depends-On: I0546a4f18e0751d209b6b7c6cd42973f25828313
Depends-On: I4924ae941b3844b39dd3f44c6986c3bf29b0d62a