Tests: memory pool unit test

Tests: memory pool unit test

Previously, the memory pool was covered only indirectly by a single
flathsh configuration.

The fix is to add direct coverage for allocation, reuse, alignment, and
mixed allocation sizes across valid pool configurations.  The test uses a
smaller workload in the regular matrix and a full stress workload in the
ASan job.

Library tests are included in the standard test target.  The added ASan
configuration omits debug memory so that the complete test target and the
pool page and cluster paths are exercised.  The library-test dependency
rule is also corrected for the new test executable.