[pty_test] Test that poll() actually sets the POLLIN flag in PollAndReadFd

PiperOrigin-RevId: 534841961

test/syscalls/linux/pty.cc

@@ -374,6 +374,8 @@ PosixErrorOr<size_t> PollAndReadFd(int fd, void* buf, size_t count,
         continue;
       }
       return PosixError(errno, "read failed");
+    } else if (n > 0 && (pfd.revents & POLLIN) == 0) {
+      return PosixError(EINVAL, "Poll said not readable but data was read");
     }
     completed += n;
     if (completed >= count) {