[T/RM] Add Bishop's effective stress tests.

ProcessLib/RichardsMechanics/Tests.cmake

.../RichardsMechanics/bishops_effective_stress_power_law.prj

...ps_effective_stress_power_law_pcs_0_ts_104_t_3.000000.vtu

...ops_effective_stress_power_law_pcs_0_ts_25_t_0.500000.vtu

...ops_effective_stress_power_law_pcs_0_ts_50_t_1.000000.vtu

...ops_effective_stress_power_law_pcs_0_ts_52_t_1.500000.vtu

...ops_effective_stress_power_law_pcs_0_ts_54_t_2.000000.vtu

...ops_effective_stress_power_law_pcs_0_ts_79_t_2.500000.vtu

...sMechanics/bishops_effective_stress_saturation_cutoff.prj

...tive_stress_saturation_cutoff_pcs_0_ts_104_t_3.000000.vtu

...ctive_stress_saturation_cutoff_pcs_0_ts_25_t_0.500000.vtu

...ctive_stress_saturation_cutoff_pcs_0_ts_50_t_1.000000.vtu

...ctive_stress_saturation_cutoff_pcs_0_ts_52_t_1.500000.vtu

...ctive_stress_saturation_cutoff_pcs_0_ts_54_t_2.000000.vtu

...ctive_stress_saturation_cutoff_pcs_0_ts_79_t_2.500000.vtu