No test coverage for anchor-center with safe/unsafe

[Scony]

As per my understanding of the spec, the safe and unsafe keywords should alter the behavior of anchor-center.
Assuming the above is true, we got no coverage for that.

IMO what we should do is to:

1. clone the tests like:
   https://github.com/web-platform-tests/wpt/blob/master/css/css-anchor-position/anchor-center-htb-htb.html
   https://github.com/web-platform-tests/wpt/blob/master/css/css-anchor-position/anchor-center-htb-vrl.html
   https://github.com/web-platform-tests/wpt/blob/master/css/css-anchor-position/anchor-center-vrl-vrl.html
   https://github.com/web-platform-tests/wpt/blob/master/css/css-anchor-position/anchor-center-vrl-htb.html
2. change align-self: anchor-center; to align-self: safe anchor-center;
3. change justify-self: anchor-center; to justify-self: safe anchor-center;
4. adjust expectations
5. repeat 1-4 for unsafe

I'd do this myself, but I'm not 100% sure if I understand correctly how safe/unsafe should work in conjunction with anchor-center.

[nt1m]

@fantasai @tabatkins