[asan][test] Don't XFAIL Posix/unpoison-alternate-stack.cpp on Solaris

One ASan test currently `XPASS`es on Solaris:

  AddressSanitizer-i386-sunos :: TestCases/Posix/unpoison-alternate-stack.cpp

It was originally `XFAIL`ed in D88501 <https://reviews.llvm.org/D88501>
because `longjmp` from a signal handled is highly unportable, warned
against in XPG7, and was not supported by Solaris `libc` at the time.

However, since then support has been added for some cases including the
current one, so the `XFAIL` can go.

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

Differential Revision: https://reviews.llvm.org/D97933