Add base class for gtests

[CAHEK7]

Gtests are using random generator and setting internal library state based on environmental variables. It may work for a case single binary-single test, but will not work for a single binary-all the tests case since the tests will interfere each other and the behavior will be heavily dependent on the test order.

We need a base class for gtests, which will cover resetting prng at the beginning of the tests (trivial, by adding prng::reset_seed(); in the constructor) and handling consistent environmental variable state but saving and restoring full state of the updated env variables (including isSet/isUset state).

[reidkwja]

currently addressed in #3321