Refactoring operator approximation tests.

src/core/terminal-colors.h

src/operators/diffusion/tnlLinearDiffusion.h

src/operators/fdm/tnlBackwardFiniteDifference.h

src/operators/fdm/tnlCentralFiniteDifference.h

src/operators/fdm/tnlExactDifference.h

src/operators/fdm/tnlForwardFiniteDifference.h

tests/unit-tests/operators/CMakeLists.txt

...s/unit-tests/operators/diffusion/tnlLinearDiffusionTest.h

tests/unit-tests/operators/fdm/tnlFiniteDifferencesTest.h

tests/unit-tests/operators/tnlApproximationError.h

tests/unit-tests/operators/tnlPDEOperatorEocTest.h

...nit-tests/operators/tnlPDEOperatorEocTestFunctionSetter.h

tests/unit-tests/operators/tnlPDEOperatorEocTestMeshSetter.h

tests/unit-tests/operators/tnlPDEOperatorEocUnitTest.h

tests/unit-tests/tnlApproximationError_impl.h

tests/unit-tests/tnlUnitTestStarter.h