Incorrect label reference: end_loop: should be end_label:

The jump references in tests 18 and 19 are using an incorrect label (end_loop: instead of end_label:), as previously discussed in the forum.

Context:
While the task specifies pairing start_loop with end_loop, the tests (refs 16 & 17) instead use start_loop with end_label. For consistency, tests 18 and 19 should follow the same pattern, but the labels need correction.