test: rework testrunner workers (#1296)

This change introduces a TestWorker that can be in a certain state,
meaning it has run some beforeAll hooks of a certain test suite stack.

TestWorker can be created at any time, which allows for a number of features:
- don't run hooks for suites with no runnable tests;
- smarter test distribution (and possibility for variuos strategies);
- recovering from hook failures and test failure by creating a new worker;
- possible isolation between workers by running them in separate environments.