Add a beacon proxy contract (#2411)

Co-authored-by: Santiago Palladino <spalladino@gmail.com>

contracts/mocks/BadBeacon.sol

@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity >=0.6.0 <0.8.0;
+
+import "../proxy/IBeacon.sol";
+
+contract BadBeaconNoImpl {
+}
+
+contract BadBeaconNotContract is IBeacon {
+    function implementation() external view override returns (address) {
+        return address(0x1);
+    }
+}