Test fixes for CAPTCHA component

- ZendTest\Captcha\ImageTest::testCaptchaImageCleanupOnlyCaptchaFilesIdentifiedByTheirSuffix
  often fails when running under parallized builds as the test directory
  gets cleaned before we're done. Added a constant to skip the test by
  default.
Merge branch 'hotfix/captcha-failures'

Fixes test failures due to timing issues in captcha GC