Testsuite: move some files for more consistency in tests file structure

meson.build

tests/decision_rules/first_order/+matlab/+namespace/y_k.m

tests/decision_rules/first_order/fs2000_cr.mod

tests/decision_rules/first_order/walsh.mod

tests/decision_rules/k_order/burnside_k_order.mod

tests/decision_rules/k_order/fs2000k2_m.mod

tests/decision_rules/k_order/fs2000k2_use_dll.mod

tests/decision_rules/k_order/fs2000k2a.mod

tests/decision_rules/k_order/fs2000k3_m.mod

tests/decision_rules/k_order/fs2000k3_use_dll.mod

tests/decision_rules/k_order/fs2000k_1_m.mod

tests/decision_rules/k_order/fs2000k_1_use_dll.mod

tests/decision_rules/second_order/burnside_1.mod

tests/decision_rules/second_order/ds1.mod

tests/decision_rules/second_order/ds2.mod

...imulations/initval_endval_blocks/histval_initval_test.mod

...simulations/initval_endval_blocks/initval_endval_test.mod

tests/stochastic_simulations/fs2000_k_order_simul.mod

...c_simulations/pruning/AS_pruned_state_space_red_shock.mod

..._simulations/pruning/AnSchorfheide_pruned_state_space.mod