Unbreak the gcc build with sendfile_test after r343362

gcc 8.x is more pedantic than clang 7.x with format strings and the tests
passed void* variables while supplying %s (which is technically
incorrect).

Make the affected void* variables use char* storage instead to address
this issue, as the compiler will upcast the values to char*.

MFC after: 1 month
MFC with: r343362
Approved by: emaste (mentor; implicit)
Reviewed by: asomers
Differential Revision: https://reviews.freebsd.org/D18934