Support multiple sources for syntax tests.

test/libsolidity/syntaxTests/multiSource/one_source.sol

@@ -0,0 +1,7 @@
+==== Source: SourceName ====
+contract A {
+	uint256 x;
+	function f() public pure { x = 42; }
+}
+// ----
+// TypeError: (SourceName:53-54): Function declared as pure, but this expression (potentially) modifies the state and thus requires non-payable (the default) or payable.