Migrate ZeroEx proxy tests to foundry (#638)

* Implement base instantiation of the ZeroEx proxy in foundry
and move TestZeroExFeature contract to the foundry mocks

* Migrate zero-ex tests to foundry

* Update copyright year

* Update contracts/zero-ex/tests/mocks/TestZeroExFeature.sol

Co-authored-by: Jacob Evans <jacob@dekz.net>

Co-authored-by: Jacob Evans <jacob@dekz.net>

contracts/zero-ex/tests/mocks/TestZeroExFeature.sol

@@ -0,0 +1,41 @@
+// SPDX-License-Identifier: Apache-2.0
+/*
+
+  Copyright 2020 ZeroEx Intl.
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+
+*/
+
+pragma solidity ^0.6.5;
+pragma experimental ABIEncoderV2;
+
+import "../../contracts/src/fixins/FixinCommon.sol";
+import "../../contracts/src/ZeroEx.sol";
+
+contract TestZeroExFeature is FixinCommon {
+    event PayableFnCalled(uint256 value);
+    event NotPayableFnCalled();
+
+    function payableFn() external payable {
+        emit PayableFnCalled(msg.value);
+    }
+
+    function notPayableFn() external {
+        emit NotPayableFnCalled();
+    }
+
+    function unimplementedFn() external {}
+
+    function internalFn() external onlySelf {}
+}