Fix issues related to inheritance in the cpp testing framework

Issue 1:

Our tests inherit from the base class test. We should define the destructor of the test class as virtual. The current implementation is not safe and may result in memory leaks and undefined behaviors. We should make sure each base class has a virtual destructor.

Issue 2:

The current code generates a warning on MacOS:

  CXX      tests/run.o

In file included from ../../../test/cppsuite/tests/run.cxx:38:

../../../test/cppsuite/tests/example_test.cxx:40:5: warning: 'run' overrides a member function but

      is not marked 'override' [-Winconsistent-missing-override]

    run()

    ^

../../../test/cppsuite/test_harness/test.h:72:18: note: overridden virtual function is here

    virtual void run();

                 ^

1 warning generated. 

The keywords override final are missing in example_test.cxx.

Definition of done:

Address the issues described above, check all CPP tests are OK.