basic/missing_capability: clean up our defines and check that our fallback is up-to-date

There is little point in #defining and #undefining CAP_LAST_CAP multiple times.

The check is only done in developer mode. After all, it's not an error to
compile on a newer kernel, and we shouldn't even warn in that case.

src/test/test-capability.c

@@ -7,6 +7,8 @@
 #include <sys/wait.h>
 #include <unistd.h>
 
+#define TEST_CAPABILITY_C
+
 #include "alloc-util.h"
 #include "capability-util.h"
 #include "errno-util.h"