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
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 <firstname.lastname@example.org>