tests/xtimer_now64_continuity: disable running test on native

The test randomly fails for unrelated PRs. Expecting real-time timing on
native is not really possible. Maybe it could be fixed but it currently
triggers many false positive.

tests/xtimer_now64_continuity/Makefile

@@ -3,6 +3,7 @@ include ../Makefile.tests_common
 USEMODULE += fmt
 USEMODULE += xtimer
 
-TEST_ON_CI_WHITELIST += all
+# This test randomly fails on `native` so disable it from CI
+TEST_ON_CI_WHITELIST += samr21-xpro
 
 include $(RIOTBASE)/Makefile.include