Support multiple sources for syntax tests.

test/libsolidity/syntaxTests/multiSource/no_import.sol

@@ -0,0 +1,10 @@
+==== Source: A ====
+contract A {
+	function g(uint256 x) public view returns(uint256) { return x; }
+}
+==== Source: B ====
+contract B is A {
+	function f(uint256 x) public view returns(uint256) { return x; }
+}
+// ----
+// DeclarationError: (B:14-15): Identifier not found or not unique.