Use same multi-source-syntax for test files

test/libsolidity/ASTJSON/documentation.sol

@@ -1,14 +1,14 @@
-// ---- SOURCE: a
+==== Source: a ====
 
 /**This contract is empty*/ contract C {}
 
-// ---- SOURCE: b
+==== Source: b ====
 
 /**This contract is empty
  and has a line-breaking comment.*/
 contract C {}
 
-// ---- SOURCE: c
+==== Source: c ====
 
 contract C {
   /** Some comment on state var.*/ uint public state;