feat: move mock solidity contracts to mocks/ folder

test/mocks/InsecureTargetBounty.sol

@@ -0,0 +1,17 @@
+pragma solidity ^0.4.18;
+
+
+import {Bounty, Target} from "../../contracts/Bounty.sol";
+
+
+contract InsecureTargetMock is Target {
+  function checkInvariant() public returns(bool){
+    return false;
+  }
+}
+
+contract InsecureTargetBounty is Bounty {
+  function deployContract() internal returns (address) {
+    return new InsecureTargetMock();
+  }
+}