meson: Rework clang-tidy integration to be done via unit tests

Instead of using run-clang-tidy.py with its own scheduling, let's
just gather a list of source files ourselves and then use that to
add a unit test for each source file that runs clang-tidy on the
source file.

We also add a bit of logic to run clang-tidy on most header files
as well for extra coverage. This uncovered various header files that
were not standalone so this commit also includes fixes to make sure
the clang-tidy tests are all green.

We can also use this in a later commit to run clang-include-cleaner
on each source file in the same way.

src/machine/operation.h

@@ -7,6 +7,7 @@
 #include "sd-event.h"
 #include "sd-varlink.h"
 
+#include "assert-util.h"
 #include "list.h"
 
 typedef struct Machine Machine;