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>

ipatests/util.py

@@ -887,3 +887,28 @@ def get_group_dn(cn):
 
 def get_user_dn(uid):
     return DN(('uid', uid), api.env.container_user, api.env.basedn)
+
+
+@contextmanager
+def xfail_context(condition, reason):
+    """Expect a block of code to fail.
+
+    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.
+    """
+    try:
+        yield
+    except Exception:
+        if condition:
+            pytest.xfail(reason)
+        raise
+    else:
+        if condition:
+            pytest.fail('XPASS(strict) reason: {}'.format(reason), False)