memblockq-test: fix incorrect assumption of pa_memblockq_pop_missing() behaviour

memblockq-test: fix incorrect assumption of pa_memblockq_pop_missing() behaviour

The intuitive meaning of "missing" would be the difference between
tlength and the current queue length, and that's how memblockq-test
assumed pa_memblockq_pop_missing() to define the term "missing", but
that was an incorrect assumption, causing the last
pa_memblockq_pop_missing() return value assertion to fail.

This patch fixes the failing assertion and adds some comments about how
the "missing" and "requested" variables in memblockq work.