[libc] fix EXPECT_EXIT suspend/timeout for darwin (#166065)

Fixes: https://github.com/llvm/llvm-project/issues/166059

---------

Signed-off-by: Shreeyash Pandey <shreeyash335@gmail.com>

libc/test/UnitTest/ExecuteFunctionUnix.cpp

@@ -57,9 +57,7 @@ ProcessStatus invoke_in_subprocess(FunctionCaller *func, int timeout_ms) {
   }
   ::close(pipe_fds[1]);
 
-  struct pollfd poll_fd {
-    pipe_fds[0], 0, 0
-  };
+  struct pollfd poll_fd{pipe_fds[0], POLLIN, 0};
   // No events requested so this call will only return after the timeout or if
   // the pipes peer was closed, signaling the process exited.
   if (::poll(&poll_fd, 1, timeout_ms) == -1) {