add ability to nest test suites

We have the same problem with structure our test cases.
I need to separate my regression test cases, release test cases and helpers inside project.
It was very helpful if we can nest test suites because it will be soon difficult to take an overview about testcases in our project.

For Example:

HelpersSuite
|-APIHelpers
|-CreateOrder
|-RemoveOrder
|-AssertionHelpers
|-CheckOrder
|-CheckCustomer
...

RegeressionTestSuite
|-CreateOrdersTS
|-CreateNewOrderTC1
|-CreateNewOrderTC2
...
|-CreateNewOrderTCn
|-RemoveOrdersTS
|-RemoveOrderTC1
|-RemoveOrderTC2
...
|-RemoveOrderTCn

ReleaseTestSuite
|-CreateOrdersTS
|-CreateNewOrderTCn+1
|-CreateNewOrderTCn+2
...
|-CreateNewOrderTCm
|-RemoveOrdersTS
|-RemoveOrderTCn+1
|-RemoveOrderTCn+m
...
|-RemoveOrderTCn+m