Added a gas test for storage costs

test/libsolidity/gasTests/storage_costs.sol

@@ -0,0 +1,24 @@
+contract C {
+    uint x;
+    function setX(uint y) public {
+        x = y;
+    }
+    function resetX() public {
+        x = 0;
+    }
+    function readX() public view returns(uint) {
+        return x;
+    }
+}
+// ====
+// optimize: true
+// optimize-yul: true
+// ----
+// creation:
+//   codeDepositCost: 27000
+//   executionCost: 81
+//   totalCost: 27081
+// external:
+//   readX(): 990
+//   resetX(): 5116
+//   setX(uint256): 20212