Added tests for rasqal_results_compatible module

- Removed duplicate variables_table field; use vt.
- Made counts into unsigned.

(rasqal_new_results_compatible): Init all offsets to -1 not 0.

(rasqal_free_results_compatible): Free map and variables table.

(rasqal_results_compatible_equal): look for non-negative offsets for
equality.

(main): Added tests