sys/log/modlog: Fix append tests

Those need to be separate tests since new logs cannot be registered
after use (nor unregistered) and thus log subsystem needs to be
reinitiliazed inbetween.

Author: sjanc

sys/log/modlog/selftest/src/testcases/modlog_test_case_append.c

@@ -198,5 +198,9 @@ mltca_run(bool use_mbufs)
 TEST_CASE_SELF(modlog_test_case_append)
 {
     mltca_run(false);
+}
+
+TEST_CASE_SELF(modlog_test_case_append_mbuf)
+{
     mltca_run(true);
 }