tests/evtimer: blacklist testing on native

The test randomly fails on `native` due to timers being not accurate but
it cannot be otherwise. So better disable it than raising fake errors.

    main(): This is RIOT! (Version: buildtest)
    Testing generic evtimer
    This should list 2 items
    ev RIOT-OS#1 offset=1000
    ev RIOT-OS#2 offset=500
    This should list 4 items
    ev RIOT-OS#1 offset=659
    ev RIOT-OS#2 offset=341
    ev RIOT-OS#3 offset=500
    ev RIOT-OS#4 offset=2454
    Are the reception times of all 4 msgs close to the supposed values?
    At    662 ms received msg 0: "RIOT-OS#2 supposed to be 659"
    At   1009 ms received msg 1: "#0 supposed to be 1000"
    At   1511 ms received msg 2: "RIOT-OS#1 supposed to be 1500"

    Traceback (most recent call last):
      File "/tmp/dwq.0.3125418833043728/ef3af88c4b3615788b164464a437df5c/tests/evtimer_msg/tests/01-run.py", line 33, in <module>
        sys.exit(run(testfunc))
      File "/tmp/dwq.0.3125418833043728/ef3af88c4b3615788b164464a437df5c/dist/pythonlibs/testrunner/__init__.py", line 29, in run
        testfunc(child)
      File "/tmp/dwq.0.3125418833043728/ef3af88c4b3615788b164464a437df5c/tests/evtimer_msg/tests/01-run.py", line 26, in testfunc
        assert(actual in range(expected - ACCEPTED_ERROR, expected + ACCEPTED_ERROR))
    AssertionError

tests/evtimer_msg/Makefile

@@ -5,4 +5,7 @@ BOARD_INSUFFICIENT_MEMORY := arduino-duemilanove arduino-leonardo arduino-nano\
 
 USEMODULE += evtimer
 
+# This test randomly fails on `native` so disable it from CI
+TEST_ON_CI_BLACKLIST += native
+
 include $(RIOTBASE)/Makefile.include