Added testing file for AVL implementation
Added allocation test for avl test
Added test for testing allocation and deleting for AVL
Added test for adding data to the avl tree
Added test for adding and destroying afterwards
Added test for adding multiple elements
Added test for adding multiple and destroying AVL afterwards
Added test for colliding inserts
Added test for finding a single element
Added test for finding multiple elements
[ci skip] Fixup: test_avl_add: The ptrs to the l/r next element should be NULL, actually
Fixed compiler errors about a const modified
Fixes a bug in the avl_find test

I also updated the documentation of avl.h as
@neithernut forgot to update it.
Changed tests to conform to master
Fixed an inverted condition
Fixed occurences of NULL and removed internal
Fixed tests to conform to latest api
Fixup: Style violation
Fix: SEGFAULT in tests_avl_insert_multiple_destroy

The use of `malloc()` caused to allocate an avl structure with
initialized `root` pointer member. At some point when iterating through
this tree of non-existing memory, it segfauls. Allocating the avl
structure with a NULL root node causes the code to allocate _real_
nodes, which _can_ be accessed.
Fix: Possible segfaults, allocate the avl structure with NULLed memory