Fix tests/test_write_cache in write back mode (#892)

Fix tests/test_write_cache in write back mode (#892)

This test could fail whenever a something (kernel, userspace) decides
to flush in between of two 2048B writes. These two writes are supposed
to be merged into a single 4906 byte write by the kernel writeback cache,
but _sometimes_ the test fails because 2048 byte writes get through.
Fixes #882

Solution here is a modification how the test works - instead
of requiring an exact aggregation of 2x2048B into 4096B,
it now writes 64x2048B and requires in write-back modes
the number of received writes requests is lower than 64 - we
can expect that at least some writes get aggregated, but we do
know how many.

Co-authored-by: Bernd Schubert <bschubert@ddn.com>