unit: Test POSIX-specific message queue impl

This unit test covers code paths of the Message Handler which are only
reached by through POSIX message queue API.

Update #3716.

spec/build/testsuites/unit/unit-no-clock-0.yml

@@ -12,6 +12,7 @@ ldflags: []
 links: []
 source:
 - testsuites/unit/tc-misaligned-builtin-memcpy.c
+- testsuites/unit/tc-score-msgq.c
 - testsuites/unit/tc-score-rbtree.c
 - testsuites/unit/ts-unit-no-clock-0.c
 stlib: []