test: Perf tests part 2. Adding Test Coordinator and base test class (#923)

Adding test coordinator.
This is the central communication coordinator for multiprocess tests.
Test runner will have a test coordinator that will act as a server. Tests will create new processes that will also have TestCoordinators. TestCoordinators in these players will automatically connect as a client to the test runner.
Each test can trigger client RPCs from the test runner (server) through the TestCoordinator to execute actions relevant to the perf test.
Clients will send test results to the test runner using server RPCs.
This is further abstracted in Part 3 of this PR with ExecuteStepInContext.

To execute these, use the "Run Selected Tests (-your platform-)" button in editor to execute these tests in separate player locally. That separate player will trigger subsequent process creation.

Each child process will self destroy if idle for too long, making sure we're not leaving process alive for ever.

Author: SamuelBellomo

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