slab metrics unit test: test_metrics_delete_basic

[TomekIdczak97]

Simple slab metrics unit test for pmem: test_metrics_delete_basic

Fail message:

Assertion 'm1_buf == m2_buf' failed: m1_buf == "STATS item_alloc 1", m2_buf == "STATS item_alloc 0"

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This change is 

[michalbiesek]

item_alloc is one of stats that is only incremented so this inconsistency is expected.
After reset when slabs/items are processed by recreate method item_alloc is incremented only if item was inserted (when it is linked)

We should wait for upstream repository maintainers to decide how to handle this situation.

[TomekIdczak97]

Failure - assertion message:
Saved:

STATS item_alloc 1
STATS item_dealloc 1
STATS item_link 1
STATS item_unlink 1

After restart:

STATS item_alloc 0
STATS item_dealloc 0
STATS item_link 0
STATS item_unlink 0