slab metrics unit test: test_metrics_prepend_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 2", m2_buf == "STATS item_alloc 1"

---

This change is 

[michalbiesek]

Ref: #42 (comment)

@TomekIdczak97
I think

pelikan/test/storage/slab_pmem/check_slab_pmem.c

Line 161 in cc90888

ck_assert_str_eq(m1_buf, m2_buf);

should be modified. Now you print only one stat which is different and there could be more than one stats different.

For the future please when you add comment in GitHub regarding statistics not directly from stdout but parse it to be more readable here:
Something like:

• Before reset item_alloc was equal 2
• After reset item_alloc was equal 1

[TomekIdczak97]

Failure - assertion message:
Saved:

STATS item_alloc 2
STATS item_dealloc 1
STATS item_link 2
STATS item_unlink 1

After restart:

STATS item_alloc 1
STATS item_dealloc 0
STATS item_link 1
STATS item_unlink 0