Little slow, but I like the results so far:
PASS: TreeSet when empty should actually be empty
PASS: TreeSet when empty should not contain an element
PASS: TreeSet when empty has no element when iterating forward
PASS: TreeSet when empty has no element when iterating backward
PASS: TreeSet with [10, 20, 15] lookup succeeds for inserted elements
PASS: TreeSet with [10, 20, 15] order is correct
PASS: TreeSet with repeated elements only contains subset
PASS: TreeSet iteration works bidirectionally
PASS: TreeSet iteration fails after tree is cleared
PASS: TreeSet iteration fails after inserting an element
PASS: TreeSet iteration fails after removing an element
PASS: TreeSet iteration still works when removing non-existing element
PASS: TreeSet iteration still works when adding an already existing element