Bug #16994338 PARSING TAP OUTPUT OF UNIT TEST EXPLAIN_FILENAME-T FAILS

Problem:

The problem is that explain_filename-t is not printing a test plan as
required by the TAP protocol. The test invokes plan(NO_PLAN) but does not
invoke exit_status() at the end, where the plan would be printed.

Solution:

Invoke exit_status() at the end.

unittest/mysys/explain_filename-t.cc

@@ -158,6 +158,6 @@ int main()
          "\"test\".\"t@0023#\"",
          EXPLAIN_PARTITIONS_AS_COMMENT);
 
-  return 0;
+  return exit_status();
 }