Not sure for the reason. But one thing I don't like is that this comctl32/msg.c (with no actual tests itself) is also shown on test.winehq.org.
That is something that unfortunately does happen with v6util, too (v6util shows itself as another test in the test list). However, I thought it better than duplicating a significant amount of code - particularly since there was already a precedent set with msg.c.
I guess it should be possible to simply move the offending files from CTESTS to C_SRCS in the Makefile, perhaps? That would hopefully stop those files registering as tests in their own right. I'll try it later.
Placing the code in a header would I be another solution, albeit in my opinion a bit of a messy one.