Tests for the large memory accesses need to be more generic

[bnaecker]

See here:

transceiver-control/controller/src/large_access.rs

Line 121 in a72631e

let reads = mgmt::MemoryRead::build_many(page, offset, len)

We're trying to test reads and writes for large-memory accesses, but we're actually only testing the read-side. This should be functionally equivalent, but we need this line to read T::build_many().