add summary message for case that minitest is used from rake task

[shunsuke227ono]

Right now, SummaryReporter #summary returns a message of "You have skipped tests. Run with --verbose for details." when you have skipped tests and you do not have option --verbose.

However, if you use rake and access minitest through the rake task, you cannot add option --verbose to rake command. Instead, you need add TESTOPTS="-v" as an option to run in verbose mode. (http://docs.seattlerb.org/rake/Rake/TestTask.html)

Specifically, when I used minitest by rake test for rails/activesupport (as you can see below), I got the message "You have skipped tests. Run with --verbose for details." but it did not work when I added --verbose to the rake command. (The same thing was reported here rails/rails#13641) I thought the message might be a bit confusing when you use rake.

$ bundle exec rake test                      [/rails/activesupport]
/Users/onoshunsuke/.rbenv/versions/2.2.2/bin/ruby -w -I"lib:test" -I"/Users/onoshunsuke/.rbenv/versions/2.2.2/lib/ruby/2.2.0" "/Users/onoshunsuke/.rbenv/versions/2.2.2/lib/ruby/2.2.0/rake/rake_test_loader.rb" "test/**/*_test.rb"
I, [2015-07-11T14:54:41.023598 #94941]  INFO -- : localhost:11211 failed (count: 0) Errno::ECONNREFUSED: Connection refused - connect(2) for "localhost" port 11211
Skipping memcached tests. Start memcached and try again.
Run options: --seed 22404

# Running:

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Finished in 9.057228s, 355.6276 runs/s, 42078.1060 assertions/s.

3221 runs, 381111 assertions, 0 failures, 0 errors, 158 skips

You have skipped tests. Run with --verbose for details.

I know it is definitely not a problem of minitest, but I guess it would be more helpful if you add an explanation of run with TESTOPTS='-v' if you are using rake just in case that user is using minitest through rake (which is maybe kind of common) :)

[zenspider]

Did you run the tests after making this change?

[zenspider]

No response. Closing.

Further, I'm not sure I agree with the sentiment of this PR. What about all the other ways that tests might be run? I don't think minitest should try to cover them all. It is the responsibility of the developer to know their tools, not minitest.