Unified let syntax to be a list of objects

test/validation/items/let/test.tum

@@ -11,8 +11,8 @@
         - let:
             name:  Let it be
             values:
-                it: $(loop_param)
-                be: <| $(loop_param) == $(it) |>
+                - it: $(loop_param)
+                - be: <| $(loop_param) == $(it) |>
 
 - loop:
     name: Cycle iterating on list

test/validation/items/lua_func/test.tum

@@ -1,7 +1,7 @@
 - let:
     name: lua_func test constants,
     values:
-      lua_func test parameter: test parameter lua_func
+      - lua_func test parameter: test parameter lua_func
 
 - lua_func:
     name: fail lua_func

test/validation/items/py_func/test.tum

@@ -1,7 +1,7 @@
 - let:
     name: py_func test constants,
     values:
-      py_func test parameter: test parameter
+      - py_func test parameter: test parameter
 
 - py_func:
     name: pass py_func