Add parallel running of tests in ZenMake

[pustotnik]

ZenMake supports building and running project tests: https://zenmake.readthedocs.io/en/latest/tests.html
But ZenMake doesn't support running of these tests in parallel. Actually it is not a big problem because C/C++ testing libraries/frameworks often support running in parallel itself. At least gtest/gmock supports this. But not all testing libraries/frameworks provides parallel/concurrent running of tests by some reasons and ZenMake can compensate it.