Today, the JSTestCase already supports multi clients for `js_test` test kind. We are going to do the similar thing in FSMWorkloadTestCase so that it can also support multi clients for `fsm_workload_test` test kind. 

Here is the current design. The JSTestCase is a multi clients test case and has a JSTestBuilder to create SingleJSTestCase instances according to the number of clients. The _SingleJSTestCase is a real test case which executes the `run_test` function and starts a js process for the js test file selected by the test suite.

In this ticket, we need to abstract the common logic related to multi clients from those classes to MultiClientTestCase and TestCaseBuilder classes and refactor the JSTestCase.