allow limiting number of concurrent runs

[KazankovMarch]

Running all test classes and methods at once consumes a lot of CPU/RAM/network, that might cause problems on weak machines with large amount of tests

We need both semaphores I added because

1. classRunsSemaphore doesn't limit method runs (there may be a lot of test methods within single test class)
2. methodsRunsSemaphore doesn't limit beforeAll and afterAll methods which can consume resources as well

[KazankovMarch]

this code is needed to avoid any overhead for those who don't want to set up the limits.

But MAYBE we should add some limits by default