Running all unit tests, I got this ones related to completionlib:

Fail: lib/simpletest/testcompletionlib.php / ▶ completionlib_test / ▶ test_get_progress_all
Mock method [get_tracked_users] at [0] -> Expected 6 arguments of [Boolean: false, Array: 0 items, Integer: 0, String: , String: , String: ] but got 7 arguments of [Boolean: false, Array: 0 items, Integer: 0, String: , String: , String: , NULL] at [/Users/stronk7/git_moodle/integration/lib/simpletest/testcompletionlib.php line 530]
line ... of ...
line 1210 of /lib/completionlib.php: call to completion_cutdown->get_tracked_users()
line 556 of /lib/simpletest/testcompletionlib.php: call to completion_info->get_progress_all()
line ... of ...

and

Fail: lib/simpletest/testcompletionlib.php / ▶ completionlib_test / ▶ test_get_progress_all
Mock method [get_tracked_users] at [1] -> Expected 6 arguments of [Boolean: true, Integer: 3, Integer: 0, String: , String: , String: ] but got 7 arguments of [Boolean: true, Integer: 3, Integer: 0, String: , String: , String: , NULL] at [/Users/stronk7/git_moodle/integration/lib/simpletest/testcompletionlib.php line 559]
line ... of ...
line 1210 of /lib/completionlib.php: call to completion_cutdown->get_tracked_users()
line 594 of /lib/simpletest/testcompletionlib.php: call to completion_info->get_progress_all()
line ... of ...

Tracing the problem down, it seems to be cause by MDL-26647