add no salshed proposer test

tests/phase0/test_process_block_header.py

@@ -0,0 +1,26 @@
+from copy import deepcopy
+import pytest
+
+
+from build.phase0.spec import (
+    get_beacon_proposer_index,
+    process_block_header,
+)
+from tests.phase0.helpers import (
+    build_empty_block_for_next_slot,
+)
+
+# mark entire file as 'sanity' and 'header'
+pytestmark = [pytest.mark.sanity, pytest.mark.header]
+
+
+def test_proposer_slashed(state):
+    pre_state = deepcopy(state)
+
+    block = build_empty_block_for_next_slot(pre_state)
+    proposer_index = get_beacon_proposer_index(pre_state, block.slot)
+    pre_state.validator_registry[proposer_index].slashed = True
+    with pytest.raises(AssertionError):
+        process_block_header(pre_state, block)
+
+    return state, [block], None