test: Use SEVERITY_MASK_IDX() to find the LOG_* mask indexes

In the unit tests and fuzzers.

Fixes bug 31334; bugfix on 0.2.5.2-alpha.

src/test/fuzz/fuzzing_common.c

@@ -167,7 +167,7 @@ main(int argc, char **argv)
     memset(&s, 0, sizeof(s));
     set_log_severity_config(loglevel, LOG_ERR, &s);
     /* ALWAYS log bug warnings. */
-    s.masks[LOG_WARN-LOG_ERR] |= LD_BUG;
+    s.masks[SEVERITY_MASK_IDX(LOG_WARN)] |= LD_BUG;
     add_stream_log(&s, "", fileno(stdout));
   }