lru.h: Adds simple tests to verify good behavior

Adds new tests to the lib include/lru.h to verify methods
insert_*.

Signed-off-by: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@cloudwatt.com>
lru.h: Fixes issue with lru_clear() + add new test

The method lru_clear() must set attribute lru_num to zero
after lru_top, lru_bot and lru_mid are reseted. indeed, lru_num
is the total number of elements found in all of them.

Also the test insures the good behavior of the method
lru_adjust() - lru_touch() calls lru_adjust every time
to balance lru_top and lru_bot by the value of lru_midpoint.

Signed-off-by: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@cloudwatt.com>
lru.h: Enforces the methods lru_pin() and lru_unpin()

If lru_*pin() is called twice, the counter will be incr/decr
incorrectly since it will count more/less pinned objects than
there is and so corrupts the balancing (lru_adjust()).

Signed-off-by: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@cloudwatt.com>