ntdll: tests/file.c: actually verify right bytes received in aio test [try 3]

[Zac Brown]

Actually send a known value and check it against the received value.
This fixes the valgrind warning:
  Syscall param write(buf) points to uninitialised byte(s)
     at  (within /lib/ld-2.6.1.so)
     by  WriteFile (file.c:559)
     by  test_iocp_fileio (file.c:549)
     by  test_iocompletion (file.c:605)
     by  func_file (file.c:633)
     by  run_test (test.h:414)
     by  main (test.h:463)
   Uninitialised value was created by a stack allocation
     at  test_iocp_fileio (file.c:503)

I have more improvements to the test, but since my bigger patch was rejected I 
figured a smaller patch would be better to start with.

Thanks to Dan Kegel & James Hawkins for feedback.

This test passes on Wine (latest git) and Windows XP.

-Zac Brown

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