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user_pass_thru test bench
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This test bench exercises the pass-thru mode to the GPIO pins
that are reserved for use by a user project for connecting to
an SPI flash.  The pass-thru mode allows the SPI flash to be
programmed using the housekeeping SPI.

The testbench is essentially the same as the pass_thru test
bench, but using the pins specified for the secondary SPI
flash.  Note that the testbench does not define a controller
on the user side to access the SPI flash (which would be a
useful thing to add to the testbench).