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.

tests/run-tests.py

@@ -2513,6 +2513,7 b' def sorttests(testdescs, previoustimes, '
                 b'check': 100,
                 b'gendoc': 100,
                 b'contrib-perf': 200,
+                b'merge-combination': 100,
                 }
         perf = {}