From Second Life Wiki
Some Residents may have seen an Avatar in Second Life called Performance Tester and are wondering 'Who is that, where did she come from!? *shocked face*'
The Performance Tester is a Linden Lab account that visits certain Locations on the Grid, to test Performance. An avatar with the surname Tester is a Linden Lab Employee, it may be a Bot or it may be a Real Person, such as a Linden doing Quality Assurance Testing!
Avatars called Performance Tester are not Likely to reply to you in chat or Instant Message.