ipatests: add context manager for declaring part of test as xfail
    
    This function provides functionality similar to pytest.mark.xfail
    but for a block of code instead of the whole test function. This has
    two benefits:
    1) you can mark single line as expectedly failing without suppressing
       all other errors in the test function
    2) you can use conditions which can not be evaluated before the test start.
    
    The check is always done in "strict" mode, i.e. if test is expected to
    fail but succeeds then it will be marked as failing.
    
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>