spec-guards: knowledge_graph declares three tests twice, gf16_uart_sim_bench collides after lowering #7383

After #7365 cleared the stale seals, the next spec-guards step on master fails (tools/check_duplicate_declarations.py):

• specs/graph/knowledge_graph.t27 declares find_entity_returns_entity_when_exists, find_entity_returns_null_when_not_exists and stats_returns_correct_counts twice (added by Implement part 2 of 2 of the empty bodies in specs/graph/knowledge_graph.t27 (7 functions) #6872).
• specs/port/fpga/vivado/gf16_uart_sim_bench.t27 has fn test_single_element_multiplication / fn test_full_matrix_multiplication beside test single_element_multiplication / test full_matrix_multiplication, which collide after lowering (added by Port fpga/vivado/gf16_uart_sim_bench.v (Verilog, 1 module) to specs/port/fpga/vivado/gf16_uart_sim_bench.t27 #7234).

Fix in the specs, not in the baseline: drop the weaker or identical duplicate tests, rename the two helper functions to check_*, reseal both on the t27c lab.