Fix hashtree:sha_test_ from timing out

EQC takes a long time, and uses gigabytes of memory to generate 1MB
random binaries, as evidenced by:

    eqc_gen:sample(eqc_gen:binary(1024 * 1024)).

We're still able to write a solid test, by ensuring that we always chunk
the binary into between 1 and 16 chunks. So instead of generating the
chunk size, we now generate the number of chunks, and derive chunk size
from that.