[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
f03d296
<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

Author: rorth

compiler-rt/test/asan/TestCases/Posix/unpoison-alternate-stack.cpp

@@ -6,9 +6,6 @@
 // RUN: %clangxx_asan -std=c++20 -fexceptions -O0 %s -o %t -pthread
 // RUN: %run %t
 
-// longjmp from signal handler is unportable.
-// XFAIL: solaris
-
 #include <algorithm>
 #include <cassert>
 #include <cerrno>