Add unit test specific to Address utils (#1251) (#1316)

(cherry picked from commit 7825caa1)

contracts/mocks/AddressImpl.sol

+pragma solidity ^0.4.24;
+
+import "../utils/Address.sol";
+
+
+contract AddressImpl {
+  function isContract(address account)
+    external
+    view
+    returns (bool)
+  {
+    return Address.isContract(account); 
+  }
+  
+}

test/Address.test.js

+const AddressImpl = artifacts.require('AddressImpl');
+const SimpleToken = artifacts.require('SimpleToken');
+
+require('chai')
+  .should();
+
+contract('Address', function ([_, anyone]) {
+  beforeEach(async function () {
+    this.mock = await AddressImpl.new();
+  });
+
+  it('should return false for account address', async function () {
+    (await this.mock.isContract(anyone)).should.equal(false);
+  });
+
+  it('should return true for contract address', async function () {
+    const contract = await SimpleToken.new();
+    (await this.mock.isContract(contract.address)).should.equal(true);
+  });
+});