Hi PiqueABoo:
My DD1 also started Y7 this week. She actually had CAT style tests (computer based) on her induction day back in July.
It's a big change and a nerve racking time (and that's just the parents) - so of course it probably threw your DD (and by extension you) to learn there was a series of tests on Day 1.
However - as Coolas quite fairly suggested - schools need to quickly get up to speed with their new pupils abilities and it often is a huge intake to deal with (ours is an ordinary comprehensive - 150 pupils from 10+ primary schools) - so mass testing early on may be logistically the easiest option.
On testing - my advice is to accept its a necessary evil. I've always taken the attitude that as long as you've tried your best, the result doesn't really matter - and often it's consistently trying your best (paying attention in class, asking questions when confused, getting help when struggling, regularly keeping up with homework, reading a bit ahead, etc....) which reaps rewards at testing.
On making new friends - it really is tough to start out in a form (?school) as the only one from your old primary school. But friends with older children who've been there and done that all say that although old primary school groups cling together at first, gradually as you start to do things in class/ outside activities with other kids you'll start to make new friends as well. One friend's daughter told my DD1 her advice was to be nice to everybody at first and if you see someone at a loose end, ask them to join you (at lunch/ on your team/ in class groups/ etc...). In a few weeks things will settle down into a routine and strange new faces will become friends (or at least known classmates). Certainly looking back now, my great friends from 'senior school' turned out to be classmates I was randomly grouped with for classes/ especially on in-class projects or through school clubs. Shared interests/ experience usually leads to friendships.
Hopefully that's the end of testing for your DD and she can relax into her first week at school and learning where all her classes are, the names of new friends and deciding which teacher she likes best/ least (endless source of conversation at that stage I find).
HTH