tests: register test-merge-combination.t as small but slow run-tests.py use file size as an heuristic for test run time. The new `test-merge-combination.t` is a small file that do a lot of processing. As a result it tend to be scheduled really late but delay the full test run by a lot. On an example test run, the one-before-last test completed 279s after the start of the run, while `test-merge-combination.t` finished 355s after it. A 76s delay. This delay can be avoided since `test-merge-combination.t` only got started 175s after the start of the run.