[asan][test] Un-xfail Posix/unpoison-alternate-stack.cpp on Solaris again

`Posix/unpoison-alternate-stack.cpp` currently `XPASS`es on Solaris.  The
`XFAIL` had already been removed in D97933
<https://reviews.llvm.org/D97933>, but reintroduced by commit
f03d2960
<https://reviews.llvm.org/rGf03d29601e0951da2c88f07d4234128e14e87870> which
was never posted or justified.

Given the `XPASS`, this obviously wasn't NFC, so I suggest to remove it again.

Tested on `amd64-pc-solaris2.11` and `x86_64-pc-linux-gnu`.

Differential Revision: https://reviews.llvm.org/D108030