Ensure tests aren't accidentially run against wrong fork

Some test directories have fork names in the
directory name. Globbing tests based on -k <ForkName>
causes tests against <ForkName> to include tests
that actuall go against <OtherForkName> only because
<ForkName> matches against the glob because it
appears in the directory name.

tox.ini

@@ -37,13 +37,13 @@ commands=
     transactions: pytest {posargs:tests/json-fixtures/test_transactions.py}
     vm: pytest {posargs:tests/json-fixtures/test_virtual_machine.py}
     native-blockchain: pytest {posargs:tests/json-fixtures/test_blockchain.py}
-    native-state-frontier: pytest {posargs:tests/json-fixtures/test_state.py -k Frontier}
-    native-state-homestead: pytest {posargs:tests/json-fixtures/test_state.py -k Homestead}
-    native-state-eip150: pytest {posargs:tests/json-fixtures/test_state.py -k EIP150}
-    native-state-eip158: pytest {posargs:tests/json-fixtures/test_state.py -k EIP158}
-    native-state-byzantium: pytest {posargs:tests/json-fixtures/test_state.py -k Byzantium}
-    native-state-constantinople: pytest {posargs:tests/json-fixtures/test_state.py -k Constantinople}
-    native-state-metropolis: pytest {posargs:tests/json-fixtures/test_state.py -k Metropolis}
+    native-state-frontier: pytest {posargs:tests/json-fixtures/test_state.py -k :Frontier}
+    native-state-homestead: pytest {posargs:tests/json-fixtures/test_state.py -k :Homestead}
+    native-state-eip150: pytest {posargs:tests/json-fixtures/test_state.py -k :EIP150}
+    native-state-eip158: pytest {posargs:tests/json-fixtures/test_state.py -k :EIP158}
+    native-state-byzantium: pytest {posargs:tests/json-fixtures/test_state.py -k :Byzantium}
+    native-state-constantinople: pytest {posargs:tests/json-fixtures/test_state.py -k :Constantinople}
+    native-state-metropolis: pytest {posargs:tests/json-fixtures/test_state.py -k :Metropolis}
     lightchain_integration: pytest --integration {posargs:tests/trinity/integration/test_lightchain_integration.py}
 
 deps = .[p2p,trinity,eth-extra,test]