Don't mark closing all output buffers as risky

[mrclay]

I propose PHPUnit_Framework_RiskyTestError only be thrown if ob_get_level() > $this->outputBufferingLevel (or allow some other mechanism to allow this).

I do agree that a test that does not close its own output buffers should be considered risky, but code that empties all output buffers seems pretty reasonable to me.

E.g. 1: A framework wants to collect output to build a formal Response object.
E.g. 2: TestCase::stopOutputBuffering() empties all buffers, why is it just OK for PHPUnit to do it? ;)

If you're amenable to the idea I can send a PR.

[sebastianbergmann]

Duplicate of #2101.