Add test sources for base contract string constructor

test/sources/solidity/contracts/statements/interpolation.sol

@@ -0,0 +1,13 @@
+pragma solidity ^0.5.0;
+
+contract Interpolated {
+    constructor(string memory a) public {
+      string memory b = a;
+    }
+}
+
+contract Test is Interpolated("abc{defg}"){
+    function a(uint x) public {
+        uint y = x;
+    }
+}