test: fix usage of FILE in unit test fapi-io
test: fix usage of FILE in unit test fapi-io

The unit test had a static FILE structure used to pass to stdio FILE *
routines as a dummy value to indicate to use the __real_xxx variant of
the mocked function. This doesn't work when FILE is opaque as the
compiler cannot figure out how much storage space is needed for a FILE
struct.

Fix this by passing a dummy pointer to a data type the compiler knows
about and casting it to FILE pointer.

Fixes: #2419

Signed-off-by: William Roberts <60148e894dabf0b83bde1d460768794cd453c455@intel.com>

tpm2-software/tpm2-tss,tpm2-software/tpm2-tss,tpm2-software/tpm2-tss