test: cleanup unit/vy_mem and unit/vy_cache tests

unit/vy_mem:
 - Remove the code creating unused lsregion.
 - Make test key_def and tuple_format global variables.
 - Replace assert() with fail().

unit/vy_cache:
 - Add missing test plan.

both:
 - Move history_node_pool to test/unit/vy_iterator_helpers.c.

Needed for #5522

NO_DOC=test
NO_TEST=test
NO_CHANGELOG=test