Update solc 0.6.x test for immutable & solc 0.6.5 (#501)

test/integration/projects/solc-6/buidler.config.js

@@ -5,6 +5,6 @@ usePlugin("@nomiclabs/buidler-truffle5");
 module.exports={
   defaultNetwork: "buidlerevm",
   solc: {
-    version: "0.6.2"
+    version: "0.6.5"
   }
 };

test/integration/projects/solc-6/contracts/ContractA.sol

@@ -9,6 +9,9 @@ contract ContractA is ContractB {
   uint counter;
   uint errorCount;
 
+  uint private immutable _a = 100;
+  uint private immutable override _b = 100;
+
   modifier overridden() override {
     require(true);
     _;

test/integration/projects/solc-6/contracts/ContractB.sol

@@ -3,6 +3,7 @@ pragma solidity ^0.6.0;
 
 contract ContractB {
   uint value;
+  uint b;
 
   constructor() public {
   }