Have the crash-test infrastructure code in test6.c read and write the 512-byte block...

Have the crash-test infrastructure code in test6.c read and write the 512-byte block containing the pending-byte lock as normal. Earlier versions did not access this part of the file in order to avoid triggering an assert in os_unix.c. But the assert() has since been removed. And not reading/writing this part of the file causes problems for multiplexor tests.

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