ccanlint: make fewer tests compulsory.

Compulsory means "malformed", we might get rid of it altogether, since
any test can mark "fail" and make ccanlint exit with non-zero status.

Now we only have four compulsory tests:
   info_exists               Module has _info file
   depends_exist             Module's CCAN dependencies can be found
   objects_build             Module object files can be built
   module_builds             Module can be built from object files

tools/ccanlint/compulsory_tests/depends_build.c → tools/ccanlint/tests/depends_build.c

@@ -13,7 +13,7 @@
 #include <err.h>
 #include <string.h>
 #include <ctype.h>
-#include "build.h"
+#include "../compulsory_tests/build.h"
 
 static const char *can_build(struct manifest *m)
 {