[Idea] Add support for nested test cases

[ThCompiler]

Add support for nested test cases

In this way you can write test case as:

RunTest(fun, 
  ts.TestCase{
   Name: "usual test",
   Args: TTA(a),
   Expected: TTV(a),
  },
  ts.TestCase{
   Name: "other test",
   TestCases: {
    ts.TestCase{
     Name: "nested test 1",
     Args: TTA(a),
     Expected: TTV(a),
    },
    ts.TestCase{
     Name: "nested test 1",
     Args: TTA(a),
     Expected: TTV(a),
    },
   },
  },
)

And support concatenate name of nested test.