by default we never run disabled tests (even if they're
explicitely listed on the command-line). We add an option --enable-disabled
which will run tests even though they are disabled, and will print, for each
such test, the comment explaining why it was disabled.
The reason for the change is when you want to run "all tests which are about
NDB" for example: mysql-test-run.pl t/*ndb*.test used to run some disabled
NDB tests, causing failures, causing investigations.
Code amended and approved by Kent.