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.

--- a/tests/run-tests.py	Fri Sep 06 11:48:49 2019 +0200
+++ b/tests/run-tests.py	Sat Sep 07 14:51:18 2019 +0200
@@ -2513,6 +2513,7 @@
                 b'check': 100,
                 b'gendoc': 100,
                 b'contrib-perf': 200,
+                b'merge-combination': 100,
                 }
         perf = {}