Provide a simple way to set problems as likely to fail.

Before building Schittkowski problems with BUILD_SCHITTKOWSKI_PROBLEMS(),
the developer can set (for instance):

  SET(SCHITTKOWSKI_PROBLEMS_FAIL 11 13 16 20 21 22 23 24 25 28 33)

All the problems listed will have their WILL_FAIL property set to true,
allowing the test suite to pass.