Migrate FlashWallet tests to foundry (#637)

* Remove contracts/zero-ex/contracts/test/TestBridge.sol

* Move foundry tests to their default location

* Wire up base foundry test and start moving
FlashWallet tests and mocks to foundry

* Move remaining FlashWallet contract tests to foundry

* Switch to foundry default cache config

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

@@ -0,0 +1,40 @@
+// 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;
+
+contract TestCallTarget {
+    event CallTargetCalled(address sender, bytes data, uint256 value);
+
+    bytes4 public constant MAGIC_BYTES = 0x12345678;
+    bytes public constant REVERTING_DATA = hex"1337";
+
+    fallback() external payable {
+        if (keccak256(msg.data) == keccak256(REVERTING_DATA)) {
+            revert("TestCallTarget/REVERT");
+        }
+        emit CallTargetCalled(msg.sender, msg.data, msg.value);
+        bytes4 rval = MAGIC_BYTES;
+        assembly {
+            mstore(0, rval)
+            return(0, 32)
+        }
+    }
+}