[tests] added test benchmark files to extdata folder

inst/examples/ensemble_example.R

inst/examples/read_from_benchmark_example.R

...data/benchmarks/Elliptic/circle_radius_1/circle_1_axi.gml

...ta/benchmarks/Elliptic/circle_radius_1/circle_1e1_axi.prj

.../benchmarks/Elliptic/circle_radius_1/line_1_lines_1e1.vtu

...ks/Elliptic/circle_radius_1/line_1_lines_1e1_expected.vtu

.../cube_1x1x1_SteadyStateDiffusion/SteadyStateDiffusion.xml

...nchmarks/Elliptic/cube_1x1x1_SteadyStateDiffusion/cube.py

...rks/Elliptic/cube_1x1x1_SteadyStateDiffusion/cube_1e0.prj

...ptic/cube_1x1x1_SteadyStateDiffusion/cube_1e0_neumann.prj

...iptic/cube_1x1x1_SteadyStateDiffusion/cube_1e0_newton.prj

...ube_1x1x1_SteadyStateDiffusion/cube_1e0_quadratic_hex.prj

...rks/Elliptic/cube_1x1x1_SteadyStateDiffusion/cube_1e1.prj

...ptic/cube_1x1x1_SteadyStateDiffusion/cube_1e1_neumann.prj

...iptic/cube_1x1x1_SteadyStateDiffusion/cube_1e1_newton.prj

...rks/Elliptic/cube_1x1x1_SteadyStateDiffusion/cube_1e2.prj

...ptic/cube_1x1x1_SteadyStateDiffusion/cube_1e2_neumann.prj

...iptic/cube_1x1x1_SteadyStateDiffusion/cube_1e2_newton.prj

...rks/Elliptic/cube_1x1x1_SteadyStateDiffusion/cube_1e3.prj

...be_1x1x1_SteadyStateDiffusion/cube_1e3_bottom_neumann.prj