Boost: Support expected failures

[nicoddemus]

Expected failures as xfail.

http://www.boost.org/doc/libs/1_56_0_b1/libs/test/doc/html/utf/user-guide/test-organization/expected-failures.html

[nicoddemus]

The arguments: --log_level=all --report_level=detailed

Produces:

<TestLog>
 <TestSuite name="MyTest">
  <TestCase name="test_failure_1">
   <Error file="boost_failure.cpp" line="8"><![CDATA[check 2 * 3 == 5 failed]]></Error>  
   <TestingTime>3000</TestingTime>
  </TestCase>
  <TestCase name="test_failure_2">
   <Error file="boost_failure.cpp" line="15"><![CDATA[check 2 - 1 == 0 failed]]></Error>
   <TestingTime>3000</TestingTime>
  </TestCase>
 </TestSuite>
</TestLog>
<TestResult>
 <TestSuite name="MyTest" result="failed" assertions_passed="0" assertions_failed="2" expected_failures="1" test_cases_passed="1" test_cases_failed="1" test_cases_skipped="0" test_cases_aborted="0">
  <TestCase name="test_failure_1" result="failed" assertions_passed="0" assertions_failed="1" expected_failures="0"></TestCase>
  <TestCase name="test_failure_2" result="passed" assertions_passed="0" assertions_failed="1" expected_failures="1"></TestCase>
 </TestSuite>
</TestResult>